When I saw this other question about one somewhat illogical aspect of spellbooks in JRPG games I remembered another somewhat illogical aspect about JRPG spellbooks: How would it make sense that spellbooks or grimoires teach only one spell?
How can it be made logical that each spellbook only teaches to one person? Like, in many JRPG games (eg. the original Final Fantasy) there's a spell shop and you buy spells for individual wizards just like weapons for individual warriors.
I mean, if the spellbook is simply conveying the knowledge of how to cast the spell then multiple people should be able to learn from one book. Thus the magic system would need something like a concept of "magical energy" that is actually stored in the book and used up by the wizard.
Incidentally, I'm interested both in answers that are about how existing games explain this, but also how this could be explained in new works. I'm starting to develop a JRPG so these ideas will help.
For example, maybe at the spell shop you aren't buying a book at all, but rather some other sort of magical artifact. Maybe something like you're buying new "ammunition" for your wand...