In any wizard's lab in any dungeon in any fantasy world, there is typically found a disproportionate amount of books. Hardly anyone can read, and the printing press has barely been invented yet. Still, everyone with half a gold piece to spare seems to have at least one big room labelled "LIBRARY" on the dungeon map key, filled from floor to ceiling with rows and rows of books. The only rational explanation to this is that books are not merely made, they are also generated spontaneously from thin air, much like fungus, maggots and vermin.
Think about it: You're raiding through some random NPC's stash and unaccountably there are all these books: The Yellow Sign, The Poetic Edda, Emporio celestial de conocimientos benévolos, Ovid's Metamorphoses. Who put them there? Where were they published? Why, no one did. For the most part books just show up. There are patterns: Books seem mainly to be attracted to more books, so that the bigger libraries are the more they grow, and new books are mainly discovered by the erudite. It is rare for an illiterate pauper to discover a furnished library containing the complete Encyclopedia of Tlön behind her cupboard, but it is not unknown.
Books almost always have authors. Sometimes these authors may even exist. They may or may not be able to recollect having written the books in question. Indeed, the act of discovering that they are the author of a certain volume may become their incentive to write it, so as to avoid embarrassment. If the book in question is deemed to be politically or metaphysically dangerous by the authorities, the author may be imprisoned to prevent him/her to write said book in the future.
The work of any curator or librarian consists mainly of scouring the shelves for new and unknown books. This is a difficult task: New books may be almost exact duplicates of existing ones, differing only in the placement of a comma or a variation in the spelling of a certain country's name. A bestiary might one morning be discovered to contain an article about a hitherto unknown creature.
Some cults and philosophies claim that mirrors are responsible for the multiplication of both books and humans and so declare them an abomination.