Are books available in the forms we know them or are they on scrolls or things of that nature? How far developed is education in your fictional world?
Knowledge, of course, is power so how do your characters use their knowledge? How do they abuse it? Does literacy backfire because it is abused?
Also it can pay to think about your favourite books and characters and examine why they are your favourites. Is it unexpected depths to a character that makes them top of your best character list? What qualities does your favourite book have that you would want to reproduce in your own? (And it is qualities you're looking for. The last thing you want is to produce what others might feel was a poor imitation but if, say, the quality you liked most was the book's sympathetic characters, then you will seek to create your own sympathetic characters but they will be set in your fictional world, your stories and be unique to you).