There is a small book-shop in the centre of Florence, not too far from San Lorenzo church. It is not fancy and you won’t find any “new releases” there, so if you’re looking for a recent mainstream novel, you’d better not waste your time by going there. But if you are a bookworm (as I am) and like to rummage into shelves of second (third,…nth)-hand books, you’ve just found heaven.
Libreria XVII Aprile is very tiny and there are books everywhere. The owner is a lovely older woman who is sitting there, reading all the time, but if you ask her about some title she will promptly stand up and will kindly assist you in any research you need.
If you don’t understand Italian, you can ask for books in foreign languages (I asked the owner and she told me she has some); you won’t have such a wide choice as in Italian, but it still is worth a visit.