Obviously the main setting is from the books, but that's just one city. Are there any other cities or towns that have official support? If not, where's a good place to find fan-made settings?
Baltimore is the most fully realized setting,
The Dresden Files RPG books Your Story and Our World cover Baltimore fairly in-depth, with locations, aspects, NPCs, and plot hooks.
Chicago is the default setting,
But it's more covered in the novels than in the game, so although there's a lot of Chicago in the game manuals there's no proper setting bible like Baltimore gets.
and you can find some good Actual Play type cities if you look.
But I strongly recommend that you use these only as examples, and have your group its own setting.
City creation is also a great introduction to FATE's ethos and mechanics. Nothing will work for your group quite like a setting that they make themselves. The process can be a great experience in group cohesion, and you'll wind up with players who are much more engaged in the setting, its people, and its problems. I cannot recommend this process and its results highly enough.
That said, please look at Baltimore and Mr. Neal's Winnipeg. They're great examples (especially when taken together) of how the process works.