So, returning to books, in your fictional settings how is literature treated? Is it accessible to all or only to the privileged few? What is the literacy rate? How are people/creatures educated? If books don't exist as such on your fictional worlds, what is the equivalent? Who would be your world's most celebrated writer and how does their influence impact on the world you've created?
After all your characters will be defined to a certain extent by how educated they are and how well read they are has a direct influence on their intelligence. So working out how your world is educated (even if it is not the direct story) and the role books play in your setting can be a useful way of distinguishing between characters (those who are well read and those who are not for one thing). A character's taste in literature (if they have one) can reveal a lot about them too.