Reposted from my London Calling thread, but I wanted to throw this book out there as a general resource, and not only if you're setting a game in London; the setting is very Dresden-like and there's plenty of ideas to steal and pilfer. If you ARE setting a game in London, there's plenty of ideas for locations, slang and plenty of juicy tidbits about the Metropolitan Police to nick.
Good stuff - two thumbs up!