If your resources and housing situation permits, you may want to pay some attention to the windows in the planned library room. It is best when they are facing only one side, preferably not the sunny side of the house. Some people opt for ceiling light access only. The idea is that you get direct light only on the books you are currently reading. Keep this in mind when you encounter home libraries with no windows at all! This design move is almost a given in shelving areas of large public libraries, but remember that you are creating a personal living space in your house. A nice solution to this conundrum is to make a wing (see picture on the left) that would be very well exposed to natural light. You can read or relax in this wing, but make sure to return the books to the shelves when you are done with them, to protect them from sun damage.
Other than these considerations, windows are always a nice way to carry through a particular style of design that you are going for - whether it's Renaissance, Victorian or Art Deco.