Most urban fantasy isn't very good (at least what I've read so far: Justine Musk, Mike Carey, some others). But if you get a chance, read Lev Grossman's The Magicians: http://jseliger.com/2009/08/28/the-magicians-lev-grossman . Although it's more straight-up fantasy than urban fantasy, Philip Pullman's His Dark Materials trilogy is also excellent.
