The Paperback Exchange is our favorite English-language bookstore in Florence. It's a great place to browse for new and used books of any sort - from architecture to literature to travel. They also have a small, but interesting kids book section. The used books are offered at a deap discount, while the new books I would say are just about market price. You can also exchange your used books for cash or credit - perfect if you're travelling and don't want to carry already-read books to your next destination.
They recently moved to a new location right near the Duomo at #4 (red) Via delle Oche. Their website has a nice map of their location, plus hours, phone numbers, photos and more.