Authors are magic users who have chosen to make a fictional world the source of their magic, written in a book they try to have access to at all times, instead of training themselves to follow the usual path of magic which involves channeled energy from the soul.
The book is enchanted to simulate the workings and goings on of a world in a universe from the rules(physics, general description of the world, starting politics, etc) the author puts down on the first page of the book, with all subsequent pages flashing with sketches and paragraphs in a dizzying display of it trying to write down and remember everything that happens within the universe of the author's book. No matter how many times you turn a page past the center of the book there always seems to be more pages to go, a showing that the fictional world is technically infinite, with excess pages piling up on the left or right seeming to disappear if you turn too many pages as if they never existed in the first place.
They can enter and exit their world at will and explore and influence it at their leisure, but they must be able to fit through the portal that appears if they were to step on their book when it is laid down with its pages open. This portal is the size of two of the book's pages, and so for the most part authors try to either be very skinny or carry a very large book so that they can fit through. Should they not like something that's a thing or that's happening in their book's world they can take a quill once outside of it and rewrite reality within their book's universe on one of the pages. What they've written disappears as things on the book's pages usually do of course, but the changes have nonetheless been made.
While they are essentially gods within the worlds of their books, there is scholarly debate as to whether the art of authors is too dangerous to be practiced by the general public, because just like how an author can go into and stay in their book, things can come out of their book and stay in the real world.
Authors have used their magic to conjure up from their books whatever they seem to require at that particular moment, if their worlds have what they need, and have even been observed to conjure loyal creatures from specific books that serve their every whim.
Fortunately there are some limitations to this ability, being that things coming out of the book follows the same requirement of needing to be able to fit through the portal the size of two of the book's pages, and should the physics under which it exists in the fictional world of an author not fit with the physics of the real world it will either not function as it does in fiction or soon cease to live or exist entirely as the fictional physics holding it together has no sway in the real world.
Authors are aware of these limitations on their own magic and so try to have worlds with physics that fit as close to those of the real universe as possible. Another peculiar quirk of their magic is that whatever books they want to use for their magic needs to be leather-bound and have within it 100 actual writable pages. The quality of the leather and the paper doesn't matter, just that those conditions be fulfilled for the enchantment of the book to work. The ink and quill used in the writing of the rules on the first page doesn't matter either, just that they are good enough to write things with in a manner that is legible to more people than the author.
'Real world' physics in the setting is basically like ours, with exception to the magic that people can do which by definition basically ignores physics anyway but I thought I'd mention it regardless. For a power comparison to another kind of magic user, a fire mage can produce fire like a flamethrower and can cause tnt-like explosions. Many others exist. There's no inherent risk or cost to using magic other than being stupid with it and it costing your life, like a fire mage setting off the explosion too close to himself and subsequently dying, because I follow that just because you can use a certain kind of magic doesn't mean you're immune to its effects and would require from you to either use it intelligently or suffer the consequences. More than one magic type can be learned, usually a shielding one along with whatever else the magic user has learned just so they can be a bit more careless if they need to be, but any more than two would likely take more years of study than most people are willing to go through.
Is it possible that authors might be too powerful for your typical medieval fantasy setting?
List of changes to be made to make them less overpowered/apocalyptic in accordance to answers, and possible self-realizations:
- The book must be able survive the presence of whatever comes out of it in order for the book to allow it to enter the real world. This prevents things like conjuring a piece of a star and destroying the world.
- Living things pulled out of a book, intelligent or not, will not be able to practice magic in the real world even if they had magic in their fictional world due to them not having souls from the real world. Soul manipulation magic might be a workaround to this but whether an author wants to involve themselves with necromantic cults or practices to grant their summons the ability to use magic in the real world(which they'll still need to learn) will be up to them.
- Nothing in the book world has a greater intelligence or cognitive capacity or learning ability than the author that made the world, but they might know things the author doesn't that is specific to living in the world in the book, like how to play a specific song or instrument the people in the book have or how to work a smithy or what mushrooms from the book-world are safe to eat.
- Each time something comes out of the book that is not the author an actual page of the initial pages disappear from the book, with up to a maximum of X-1 instances(to make it so that the first page stays) of things being able to be pulled out of a book for any period of time, X being the amount of pages the book was originally made of. Putting something back into the book returns the page and allows for using it for another purpose. This does not hurt the world's simulation or suddenly cause certain areas or things to stop existing in the book world, but this should at least somewhat limit the infinite wealth issue. For things that can't exactly stay separated from other things of its kind that was summoned and would be intermixed(like sand), any kind of approximate mass of the summoned things returned to the book will do and needs not be the actual original sand or whatever that was summoned from the book. This should also help curb the stockpiling of too many explosives or other items of such 'I win because I have infinite X' scenarios.
- Authors need to be aware of the physical locations of things inside of the book world in order to summon them. This would force them to need to experience and travel the world in their book in order to find that which they might want to summon. Unless a living thing is caged up or more or less in the same place at all times anyway this may also make summoning them less reliable. This would also force them to mine for gold or whatever other raw resource in order to know where it is if they want to summon it in the real world, if their world even has it in the first place. Manually making sure the gold is present is an option but would still require knowing where it is to summon. Sure they might be able to manually make sure it's easily attainable but this would still require them to write its physical location in by using relative reference points in their writing that would only be attainable by being in and experiencing their book world, which would also need to be survivable for them in order to do this.
- Just like the real world, the book world has a chaos factor that would make the exact simulation of the real world for future prediction or counterfeit or other such reasons hard to pull off. With enough books made with the same set of rules with the purpose of simulating the real world the author might eventually come across a book world that gets things right but the odds of that are slim and most sane people would probably give up at some point.
- There is a degree of skill and knowledge involved/required with writing the rules of the world on the first page. Failure to account for certain things might be harmless or might prove lethal to the author entering their book due to many things that would fit the bill of 'a world with our physics and with life', as the world might not have oxygen for example and the author wouldn't survive there but it would be perfect for the simulated creatures there. Since the first page is limited it can be hard to account for everything without using an excessively huge book, so authors would need to choose their words carefully and use the space efficiently if they want to feasibly carry around their book. Edits can be made to the rules if they realized they went wrong somewhere but it would waste valuable writing space on the first page.
- Books pulled from a book world cannot be used for the enchantment that creates an author's book. Just in case anyone is thinking of this :)
- The 'still being able to pull casters out of your book by putting them into the world as babies' I view as a valid exploit that a morally questionable author might do. People would notice the missing babies and would try to find them or do something about their disappearance, like dealing with the author if they suspect one.
- Books will not allow things to be pulled from them if its presence would lead to the book's destruction as a result of its breakdown due to incompatible physics or due to the function of an object not being possible in the real world, like the breakdown of a star genesis or containment device. Hopefully now there'll be no more stars from the books.
- Author books are soul-bound to the author and only allow one per author like a unique lock and key system. A different book can be obtained by removing the enchantment on an author's book and turning it back into a mundane one. This can be used to rewrite the rules of an author's world if their previous one ran out of space or wasn't desirable, or increase the amount of pages in the book. Should an author's book be turned back into a mundane one, anything they have summoned(is still present in the real world) will cease to exist. An author that is killed will also turn their book back to a mundane one.
Still thinking about ways to deal with the other things pointed out in wizzwizz4's answer.