You may find the following website useful for local recommendations: http://www.studentsville.it/index.htm
Right downstairs from our school is one of the most comprehensive and convenient bookstores in all of Florence. Edison Bookstore (Piazza della Repubblica 27/r) is open Monday through Saturday from 9 am until midnight and Sunday from 10-midnight. This is an excellent place for late night browsing, and the store includes a cafe. Edison not only has a good English language book / magazine selection, they carry other items as well. Drop in on your way out of class and discover their collection.
If you are specifically interested in English language books, another wonderful nearby option (both new and used books) is Paperback Exchange at Via delle Ocche 4/r (open 9-7:30 Monday through Friday and 10:30-7:30 Saturdays).