In a JRPG-like setting; Spells are chanted, mana is consumed, and spells can be learned through grimoires or spellbooks, how would it make sense that a spellbook teaches only one spell?
If it is like cooking, like say, how to bake a cake. Why would it take a whole book to learn? How could I make sense out of this?
It's necessary because each spell requires a special way of thinking about the aspects it controls. If you want to use a certain spell from what is commonly called Fire Magic, you have to understand where you get the energy, what to do at what stage of the spell to not burn yourself, how to direct that energy towards your target, how to release the energy at the correct time, how to give the energy back to the endless energy around you and in yourself, etc. You have to understand how the energy interacts with other forms of energy, with your body, with your soul, with other lifeforms. You have to be aware of the consequences of losing control for even a second. You have to be aware of your responsibility.
You have to understand that the Fire Magic is not something inborn. It is something you need to train. Very often and for a long time.
The book teaches you not just words, phrases, intonations, rituals, gestures, materials needed, preparations, shortcuts, best usecases, worst usecases, as @Innovine pointed out the history of the spell and its most important users and authors with their most important accomplishments and failures, traditional usecases, experimental usecases, preferred tactics aforementioned individuals used or thought of and many more things that are directly necessary to cast a spell in battle - it teaches you how to live your life so that you have the right attitude to be able to cast that spell. Daily rituals. A certain way of thinking. Behaviour towards the energy that surrounds you and the lifeforms you meet on your way.
Magic is not just "take x energy and say Fire". Magic is a lifestyle. And each magical spell teaches a different lifestyle. So different that to truly understand it you need to read and understand a whole book about it.
Even if you know seemingly similar spells you have to understand a lot of different things for a new spell. Just because you know the Fireball doesn't make it easier to learn the Fire Wall. Maybe a bit, but there is not enough overlap to write a generic book about The Basics of Fire Magic with additional booklets about the Fireball and the Fire Wall.
Books of magic are magical, if not appropriately separated the spells have a tendency to interact in an uncontrolled manner. The more powerful the spell the worse it gets.
While it's relatively safe to keep household spells all in one book, cooking, fire lighting and cleaning spells aren't particularly dangerous, maybe your broom will leave trails of sparks round the house or your dinner might be really clean, but if you start putting major summoning spells into one tome you never know what you're going to get.
Enchanted leather bindings are critical for isolation, sometimes with metal plates inserted for the more lively spells. Scrolls are acceptable but must be vellum, not papyrus. Card or paper bindings are frankly dangerous.
In order to cast a spell, there are specific time- and location-dependant pieces of information you need to use that form part of the invocation.
While you could compute these parameters by hand each time, it's far quicker and much more practical to look their values up in a book of tables. In real life, in the 1600-1900s, people used books of tables to calculate logarithms, trigonometric functions, random numbers, and many other things.
For example, the book Logarithmic Tables: Containing Logarithms to Numbers From 1 to 120,000, Numbers to Logarithms From 0 to 1.00000, to Seven Places of Decimals is, unsurprisingly, a 252 page book containing tables of logarithms for 1-120000 to seven decimal places.
Or, as another example, A Million Random Digits with 100,000 Normal Deviates contains, as you'd expect from the title, a million random digits with 100000 normal deviates, for all your twentieth-century random number generation needs, and runs 628 pages.
Special-purpose books of tables also exist, for example Dudley's Handbook of Practical Gear Design and Manufacture is 862 pages, and includes a number of tables describing various aspects of sizing gear geometry (e.g. the dedendum angle or root clearance) for particular purposes.
In a similar vein, the book Fireball needs to contain tables for computing the required mean etheric alacrity, sub-pentacular glyph angle, Hureian correction factor, et cetera needed to correctly cast a fireball at any particular time of any particular day of the year, in any wind conditions, at the ambient arcanomagnetic flux of your location, et cetera.
Some additional features of this model:
Spellbooks are designed so that any sufficiently skilled mage can use them to cast the spell. But some spells depend on properties of the caster - e.g. the mage's height or bloodlines might factor into it. So any spellbooks intended for general use will need to include the information needed to adjust for the user. Part of mastering a a spell would be making your own version of the tables, configured for yourself.
Also remember that tables were not the only way of calculating logarithms. For fast, lower-precision computation, a slide rule or other aid could be used.
Note that while most real-life slide rules are marked for logs, trig, and multiplication, a slide rule can in principle be made that computes any function. And they don't need to be the stereotypical long flat rectangle shape either.
While the extra precision in a tome is needed for it to be general purpose, a slide rule could be good enough if specialized for the specific caster.
So, the process of mastering a spell goes like this:
To cast a spell you may not need all of the information on every page of the spellbook, but you do need some of the information on some of the pages, and it's not entirely possible to predict in advance which pages you'll need.
Thinking outside the box here: spells are only a few magic words each, but they're buried within 300 pages of meaningless gibberish for security reasons.
Because they are flip books. You cannot write down the spell, for security reasons, or because the scribble/book printer automatically would unintentionally perform the spell by themselves. (May be important for dangerous spells, such as ‘blow up the house’.) To read the spell, you have to flip though the book, which causes glyphs (like half-letters) on alternating pages to form the words. This includes some kind of security by obscurity to the non-aware, or maybe androids that try to OCR-read the book, as well. By the way, it is possible to have text scrolling or animated scenes (how to move the magic wand) this way.
Justification for something like this is pretty easy I'd say.
Recipes for even complex food dishes are simple/short. But imagine that the book had to describe, in excruciating detail, the process of farming, cultivating and gathering the required ingredients, and perhaps even the properties and the histories of each of those ingredients. That's going to be one HEAVY book!
Weather you want a spell book to be like a recipe-book/instruction-manual, or some kind of magical artefact that simply bestows spells upon a reader, if a spell is the manipulation of the very fabric of the universe/reality then even the seemingly simplest of manipulations could require a very profound understanding of very complex abstract concepts. And for the spell to be truly complete, those concepts may need to be captured in excruciating detail.
To cast a fireball, the user may need the magical equivalent of an encyclopaedia worth of knowledge of thermodynamics and all of the interacting forces. Not to mention the initial magical know-how required to interpret the writings or harness the required basic mana/energies (or however magic works in your world).
How big does a book get if you try to squeeze in a few extra highly complex understandings woven together in tapestries of wisdom, accessible by understanding only to the select few that can fully comprehend them?
Quite big.
The key insight you need is that the phrase used to cast a spell is not the spell.
If a magic spell consisted of a few words, or a couple of lines like a recipe for cooking, every farm boy would be a level 20 wizard.
The reason that magic is comparatively rare in most fantasy worlds is that it is difficult. It takes years of study to become a wizard, sometimes decades. That in mind, the assumption that spells are in any way simple does not make sense.
As stated, the phrase used to cast a spell is not the spell. It is just the command words to invoke the spell.
In computer terms, when you enter "firefox.exe" on a command line, or click its icon in a GUI, this action is just the command you utter to start the browser. The actual browser is far, far more complicated than that.
Likewise a spell book. It contains the actual spell, like the source code of a program. Learning the spell is like compiling the source code. Then casting the spell is like typing "firefox.exe" (verbal component) or clicking an icon (gesture).
And that's why a spell book is big and yet contains only one spell. Because it details everything you need to know to cast the spell, which is a lot more than just the command phrase.
This also neatly explains why in some systems you need to memorise/prepare spells - that is basically the equivalent of the compiling process.
Why would you assume they are the same as cook books? I always think of those things the following way if I encounter them for example in a game, but I never used spellbook based magic myself. Combinations of those are of course desired.
There is a lot of literature on one specific method or subject or piece of technology in science or computer science and so on. If you dive deep into a particular subject, things might become complicated and you need time and room to explain all the details. I imagine magic is very complicated since nobody I know can do it. One could also put the entire history of a spell and famous users or uses and so on into a book.
In computer games, we break down what might be (virtual) reality into some simple simulation. The book "iceblast" might describe several variations of the iceblast, but for simplicity (one needs to operate the game with some controller and very quickly), only one, maybe averaged, iceblast is implemented into the game. The in universe book then contains how many iceblasts you want. Think of it as a cookbook on a particular subject, for example soups. One soup in the game, but many soups in the real world.
Long Books might not be practical. So you want to teach one spell, why not do it in the form of a small booklet or scroll that has maybe a couple of pages? Why should every book be a thousand page long encyclopedia that your magic student has to browse through? When you where a student, did you ever receive a couple sheets of paper from your teachers with some sort of information on a particular topic or did you always get massive books form them where everything humans know was collected?
So you invent/discover a spell. You want to sell/distribute it. What do you do? You don't start a major project and collect all other spells known to man. You just communicate what you've got and be done with it.
I also believe that one could write one (maybe not that large) book on one particular dish with its history and variations and so on, but since most humans don't want to eat the same thing over and over again, one usually doesn't buy those books. But they even do exist. Check out a local dish that's good and you might even find a couple of books written about it.
Why do you think a grimoire consists of pages of paper?
Because every page only contains ONE RUNE!
The shape of the rune must be drawn in a precise manner, lest the spell's power is diminished, or worse, backlashes to the caster in a spectacular manner.
Besides that, each page is thick to allow the magic ink to properly seep into the pages and withstand the aging.
If a spell requires 20 runes to be properly cast, then you can see why a grimoire can only contain one spell. Or maybe, one half of a Summon Meteor spell?
A single spell could be complex enough, with enough actions and thoughts that must occur within a short amount of time, that it requires a regimen of training to be able to accomplish. That regimen is in the book.
Suppose I have a spell that builds a bridge across a chasm. Any inaccuracy in the wizard's imagination makes it collapse.
Suppose I have a spell that raises a castle from the ground. Any slip in the words results in a useless pile of rubble.
Suppose I have a spell that can cut a forest of trees, split them into boards and fly all of that into place as a nearly impenetrable barricade: Slip up and you have a pile of trees, or worse, a forest fire surrounding you, or you (the wizard) are impaled by your own magic.
Some spells are easy, but big spells are like playing a piano concert flawlessly: They can take months or even years of practice for the wizard to learn and embed in "muscle memory", and the book shows you exactly HOW to practice the big spell without killing yourself or destroying your house.
I think your missing one very important aspect of books.
They come in all shapes and sizes. Spellbooks are not 300 page leather bound monstrosities, they are in fact the magical equivalent of the leaflets you pick up at your nearest tourist information, anywhere between 4 and 20 pages depending on the spell.
Spellbooklets if you will.
You might struggle to use this reasoning if referring to a tome as that word most specifically refers to a large book, but if you combine this idea with some of the other answers (a variety of complex information is required to learn a spell beyond a mere incantation to be repeated, or each page contains single phrases or runes in large writing for ease of reading).
Conveniently this also explains why you can carry a lot of them and makes it more believable that they are consumable items that can only be used once.
If you expect to sit down, read a spellbook like a textbook, and learn something about magic, you've got a long way to go. The first lesson any student of magic learns is that you can't learn how magic works or how to control it in the way you'd learn to understand and control mundane things.
If you look in a spellbook, all you'll see is incoherent gibberish, scribbles you can't seem to quite focus your eyes on, like optical illusions without the trickery. Just like some illusions make more sense when you learn to relax your eyes to focus at a point beyond them, to receive the knowledge contained in a spellbook you must relax your mind and allow the imagery to become your thoughts.
The goal isn't to make sense of the lines, because they already make sense - they are a pure expression of the very essence of the spell they describe. Instead of conforming their wisdom to your mind, you must form your mind to their shapes and motions. You must memorize every subtle detail of their intricacies and visualize each page as part of a larger, super-dimensional sculpture beyond space and time.
In so doing, and with singular focus, your thoughts create the ebb and flow of energy throughout your nervous system in such a way as to channel the effects of the spell into being. There is no way to verbally instruct this process - the signal patterns your brain must form are beyond description or control. There is simply too much information. Should a madman actually attempt to write a treatise on a single spell and live long enough to succeed in expounding only the necessary details of its nature, he would fill a library.
So really, it's amazing that a single book is enough at all.
If you take physics, it is similar. The laws are extremely complex. Yet we use them without ever knowing. We use them in tools and matter. These things just "know the stuff". But if you would to construct something like your own matter and dictate it laws to obey, that needs lots of knowledge ( quantum mechanics, chemistry, thermodynamics, mathematics...)
Now if you give the mages a gift to "get whatever they can imagine", but make them do it from scratch (from "atoms of magic") then you need a whole grimoire just not to kill yourself in the process. From this point, you can even scale the difficulty by demanding some complexity ( for example the level of proteins https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3032381/ - just a random paper about problems and difficulty scale of these little beasts) you can give your magi a problem worth of many books and half of live.
The words the spells are made of could be alive in some form. They need a whole book of lesser, regular words they can read and feel superior to. They would burn themselves away if they just existed on a scrap of paper (the're better than that). If you put 2 spells together they just fight and either one removes the other, or the whole book goes up in flames.
One spell may have many variations in application, for example, a slowing spell:
May be broadcast affecting everything but the caster
May be broadcast affecting everyone but the caster
May be focused on an area
May be focused on an individual, or a pack of animals
May be focused upon one object
Additional rituals may be required to act upon flowing water, rising water, falling water (rain), wind and fire
Other variations are necessary for slowing factor and duration
Surroundings also make a difference, confined and open spaces, and whether the subject is already magically affected, or for a remote subject.
Basically, although there is only one spell, there is much that must be learned to cast it effectively in all circumstances.
OR
Why can't you have a one page book?
You may make the spells either complex enough to justify "one book per spell" attitude or introduce some sort of pathological capitalism where sellers just divide books of spells into separate books for every spell to earn more selling them. Or just introduce some curse for ones printing wrong combinations of spells together.
To make spells more complex you may want to make them something more than "just chant this word loudly". Like explaining the emotional state you have to be in, the intonation, magical rituals that you have to do in order for a spell to be effective etc. Then all these things may require preparation: you need to meditate to alter your feelings and mind state, you need to prepare objects used in the ritual. Additionally the book may also contain some justification for why it is done this way and "scientific" ("magical") explanations. This way you get enough content per spell to make it reasonable printing one-spell book.
The reasoning behind that is matched with how the spellbook act. If, for example, the book is destroyed in the process then the book doesn't really exist. It's a manifestation of the spell, the essence or the idea of it.
The reader is actually enchanted with the spell as items are enchanted with runes or buffs. That's also explain why the books are so rare and pricy.
The second option is that a book is real, written by someone and it can be passed through generations. Then the idea behind one spell per book is that when writing a recipe for a cake you don't make a cake. But when you write a spell you are making the spell. Kind like placing a rune. It's active but you can read it and learn what type of rune it is and so on.
So for the safety of the writer, reader and the knowledge you don't put two spells in one place. Or you don't put different spells together. Fire spells can be written in one book (assuming that their power is not stackable). But putting fire and water spells next to each other would render them "broken" over time.
They would act on each other in a way that some words, incantation, hand or wand movement would disappear from the page. And they would do so until one of the spell stopped being a spell.
The grimoire doesn't teach a spell in the same way a text book does. The book actually doesn't say anything at all about magic or casting or anything related to it. It was just a book of stories. The more you read the book the more of the book you can see\understand\read. Only by reading it over and over would you be able to eventually see and read other parts and only by reading the entire book multiple times would the potential for a spell begin to imprint itself on your conscience mind.
It might be a book of gibberish or a book of fairy tales. Its the act of reading the book is what is required to learn the spell. Gradually the act of reading the information programs your brain to enable it to cast the spell. The more often and the more times you read the book the stronger your ability to cast the spell. Once its cast the energy potential built up by reading those specific words in that specific order is expended and must be built up again before it can be cast again.
The ability to write a book to teach a spell is a ability and artform all its own. Perhaps the author of the book can't actually even cast the spell themselves. Like the author of a musical symphony may not be able to play every instrument but listening to the symphony he writes for all those instruments invokes images and feelings even though in reality it's just a series of notes that actually have no emotion or imagery of their own.
"Evil" spells have dark stories to learn a truly hideous spell the composer uses truly evil stories that gradually effect the reader as they read the stories again and again. "Good" spells likewise have good stories that likewise effect the reader. Either way being forced to read a given story again and again to imprint the energy on the caster is going to have effects on them.
Easy spells might just be a poem on a single sheet that imprint themselves easily. More complex spells might be multiple volumes that each take weeks to read so to build up the potential for such a spell might take years or even a lifetime of reading gibberish that gradually drives most would be casters insane.
Spells are hard.
I mean, I can show you a fast fourier transform equation. But knowing how to use it would require a decade of mathematical background and study.
Spells are the same, but each and every spell is its own entire discipline of magic.
Casting a spell is a mental effort similar to tap dancing a complicated improvised routine matching a new partner, while doing slide-rule calculations on how much support your suspension bridge needs given cross wind harmonics and expected load, while justifying the cost:benefit to a hostile accounting agency.
And every single spell is as different as theology is from chemical engineering.
Now, every spell has more primitive versions of itself. There are a series of fire spells that go from being able to light tinder, all the way up to a meteor storm. These are related, and their development was in sequence.
Sadly, moving from many related spell to another has a large gulf of "doesn't work very well". Optimized modern spells that are easy to cast have a whole pile of improvements. Back in the day, to prepare to cast a single spell, could spend a week doing rituals and self mutilation.
Over time, more efficient ways where found to evoke certain effects. The 'ur fire' style of spell is still used during fire spell research (where you have to 'go back to first principles'), but fireball, meteor storm, flame arrows, burning hands and ignite are all specialized variants. These variants require complex, exact, distinct bits of "choreography" compared to each other to get the effect to come out as desired and as cheap as possible.
So for a set of fire spells, knowing other fire spells help, but part of each book is teaching you to break the habits of your other fire spells so you don't mix the approaches and end up with a fizzle.
Magic lets you break what otherwise seems the rules of reality. But it isn't easy.
Each grimoire contains two spells.
The fireball spells requires a 10,000 word chant - far too big for use in battle. So the fireball grimoire contains the prepare fireball spell, a 50,000 word chant that you cast by reading directly from the grimoire, and which prepares the fireball spell for casting and implants it in your mind, so that you can fire it instantly. Of course, the mind of a wizard can only hold a limited number of spells at a time.
This is (my understanding) of how the spells work in D&D, and it seems relevant to you - the spells there require 1 page per spell level to store in the spellbook, you can cast them directly from the text without expending one of your prepared spell slots, it takes 10 minutes to prepare a spell...
How about a twist on Selenor's answer:
In the modern world things stand upon the shoulders of that which came before. It's an interconnected whole that you learn as pieces and put together. (A simple example: Lets look at something that is accelerating. The equation for it's distance is the integral of the equation for it's velocity which is the integral of the equation for it's acceleration.)
However, magic dates from a time before we understood that there were a fairly small set of fairly simple rules underlying everything and that the complexity we see is the result of the combination of them. Thus while we might picture the Fireball spell as conjuring up some fire, containing it and projecting it to a target location that's not what the book does. Rather, it teaches you everything you need to know to make that ball of fire appear, without building upon any other magical knowledge the wizard had.
Compare alchemy and chemistry--where the alchemists do not recognize that two different means of synthesis can produce the same end product.
Pretty late to this question, but I think there may be a more straightforward answer here that I haven't seen mentioned yet.
Put simply:
Just because a book contains a bunch of spells doesn't mean they're USEFUL to you.
I'll give some possible scenarios, and back them up with real world analogues.
Scenarios
Imagine: you're a professional battle mage. You've been through your fair share of scraps, you cast a mean fireball, and your magic missile is on point.
After battling a dark mage, burning his mansion down in the process, you raid the scorched husk of his library. You pick up the only unburnt grimoire in the place... and it's full of cooking spells. You flip through it, and hey, there's one spell on carving a Thanksgiving turkey that you could adapt, but that's pretty much it. WELL GUESS WHAT! You just learned Knives of Orc Slicing.
Or perhaps after your latest quest, you go to the equivalent of the library of Alexandria for some light studying (you're a magic nerd, after all). You pick up a book called "Combat Conjurations, Warrior Wizardry, and Strategic Sorcery: Practical Applications" and start reading. You get 10 pages in and realize it's way too simple for you. You flip through, and realize you know all but one spell in this book, and probably better than the author. But hey, might as well pick up that one spell as well.
Since that didn't go so well for you, you move to a more theoretical tome called "Military and Para-Military Research Frontiers Pertaining to the Maximal Disruption of Ergodic Systems Utilizing Thermodynamical Processes in Mana- and Time-Limited Situations." This time, you get 10 pages in and realize the 'simple' examples are way over your head. You go checkout some textbooks on theory (with no actual spells in them) to get the grounding you'll need to tackle Research Frontiers and spend the next 12 hours brushing up. Upon concluding your refresher, you return to the tome and find you're now able to understand one more spell the text describes. Mentally exhausted, you give up for now.
Real World Analogues
I'm a mobile app developer, and I can do most things in iOS and Android reasonably well – much like the battle mage can do battle magic reasonably well.
As in the first scenario, if I read a book about a web framework, I'm probably not going to get much out of it. Maybe a pattern or two, possibly a new way to structure things.
The second scenario corresponds to me picking up an "Intro to iOS Development" book. I'm a professional, so I know the basics. Perhaps this book was written recently, so there's one feature I wasn't aware of, but otherwise, not terribly helpful.
And I think everyone has experienced the third scenario in some form. Advanced topics are, well, advanced, and if your experience so far mostly isn't, it can be hard to get through. Moreover, it can be discouraging when you do a bunch of research only to find you barely approaching the level the author is writing at. You might learn something, but you're certainly not going to effortlessly absorb all that knowledge.
To return to the the question text: if it's like a cookbook, how come you only learn one spell per book? Well, if you already know how to cook a bunch of things, any random cookbook might not teach you much.
The spellbooks are magical collections of memories of performing the spell.
If someone tried to describe, in words, how to use a magical spell to you, you probably wouldn't get it. There's all sorts of special terminology that's different between different teachers and different schools of magic. Unless both you and the explainer had the same teachers in the same order, teaching the same spells, you'll get confused by some term being different and wind up blowing yourself up or rotting off a limb or something equally unpleasant.
To teach, show the student what it's like to cast the spell
Rather than either hoping the student understands what the words are trying to say, or attempting to redefine each word in the definition of the spell, the spellbooks are instead collections of memories of different people casting the spell, in different conditions.
One-use device
Many spellbooks are one-use devices, similar to how in many RPGs spellbooks can only be used once and then are consumed. If you want to teach three people the spell you'll need three copies of the spellbook.
A spell is a merely a complex component composed of many complex components. Spells are layered and stacked, from the most basic roots to the most advanced reality-bending magicks.
Simple spells may only require a few key components. A fireball, for example, might require components that extract key elements from the air, components to bring these components together, and components to drive some form of reaction. A component to extract oxygen from the air might be composed of multiple components that deal with splitting/separating the individual elements.
A mage/wizard would be able to draw out these components in a way that best fits their own theme. A dark evil wizard might find that certain components can be formed within blood allowing specific uses of blood magic - The actual lines of blood that he draws may be composed of several layers of component spells already. Another wizard may write out / carry runes composing of key components that allow them a variety of uses.
A grimoire/spellbook/tome would teach all of the required components necessary to perform some spell. Other books might teach broader theory, layering theory, or theories on producing automated spell matrices to channel through.
The bonus here is that a sufficiently talented caster can then work with these individual components-of-components to produce their own spells and new magicks. They might learn a trick that allows them to take a component from a Fire Wall spell and apply it to water instead.
Additionally, this would offer a way for one-time spells. Instead of detailing the exact intricacies of how these components are formed and behave, the components are simply laid out in a complex matrix; potentially obfuscated by the mage/wizard to be very difficult if not impossible to properly decipher. The mage who purchased such a book would simply put in their own power/energy/magic, and it would run the complex system of components, burning the "circuitry" in the process.
I like to think of magic in my stories as being something similar to a computer program or any other kind of algorithm. While the idea behind an algorithm is simple (a sequence of steps that need to be done in order for something to happen), even if it takes, say, 500 lines of code to write a certain program (and that would be a reasonably small program), explaining what you've done is very difficult, and explaining every little concept behind that program can be very well enough for an entire book. Now, Magic would involve manipulating energies and laws that are amazingly complex, and any misstep could lead to "bugs" that could potentially kill a lot of people. So it's entirely reasonable that you would have an entire book teaching a single spell.
There have been some very interesting answers about the interaction and complexity of magic. But here's something a bit more mundane.. economics.
Think of it as if you were in the business of making these tomes. If you bundle the spells up into a single book, you will have to discount it simply because you will be including spells that your potential customers might already know or not be interested in. You would make more selling the spells individually.
It depends on the complexity of the spell.
If you think of spells like computer programs (with potentially millions of lines of code). A well rewritten and complex spell could include many of the most common conditions and variables required to implement it.
For example:
The book may go into formulas and theory behind the formulas which are all very critical to the success. At the end of the day, this type of book requires a great deal of studying but may not require a very long chant to perform the spell... or it might require months or even years of preparation.
Take a look at the "The Sword of Truth" series by Terry Goodkind. "The Book of Counted Shadows" was essentially exactly the kind of single spell book that you are describing.
The spell book itself is magic, and actually contains some of the magic (if we could quantify that somehow) for the spell.
To put two spells into one book would cause their respective magical fields to intermingle and taint each other. Best case scenario, they ruin each others' purity and the spells become less efficient / effective, at worst, they can't be compatible with one another, and rupture the fabric of reality around the book perhaps causing minor accidents (bursting into flames or corroding the things around them or something).
The covers of the books contain special magical insulation to keep the magic from leaking out.
That only leaves the matter of the content required to fill an entire book. You could use one of the other suggestions above, or maybe the spell doesn't need the entire book - the rest of the content is just the ramblings of the wizard who created the spell, interesting facts, perhaps the theory which lead to how the spell was created, etc. We've to create an entire book anyway we might as well put something in there. So doing this might have become a tradition, respected by those nerdy intellectual wizards...
I would implement a 'grimoire requirement' thusly:
Oh - and some form of magical cryptography, so that the book can be verified as genuine. That would probably go at the front or in the covers, depending upon how bulky the runecraft or whatever would be... And maybe a magical 'lock', which explains why they're single-use. Once you do the purchase, it's bonded to your blood or soul or something.
Additionally, the Discworld answer is always a good one (whether it's "spells fight" or "knowledge = power = energy = mass (times c²) = spatial distortion"). It also explains how they all fit in one's backpack or into a tiny store - they're literally warping space to fit.
Why not have spellbooks in one language with hundreds of spells? Because none of the languages are normal speaking languages. In fact, these arcane languages are essentially inherited by birth and innate to the individual, but in such a diverse way that the only practical way to ensure spells are usable by as many people as possible is to translate every spell into each of the few hundred compatible languages. Conveniently enough, there is a spell that does this translation process, so all you really need to do is write the spell in your native arcane language and then run the translation-copy spell on it to fill in the rest of the pages.
Why not write it in "common"? Magic doesn't work in common. There's no canonical pronunciation of the spells because it's a manifestation of pure thought. Much of the process of learning to become a mage involves discovering your innate arcane language and learning to understand the associated texts. Basically you just have to look through spellbooks until you reach a page that you magically understand- but even then you need to understand certain core concepts to cast spells. Mage schools help you understand those concepts.
This can also explain why mages might be able to learn some spells but not others- not every spell can actually be translated into their innate arcane language, perhaps because of the language's inability to convey certain concepts. So you might pick up a spellbook, try to find a page you understand and if you can't, you can probably assume you are totally incapable of casting that spell.
