Book Four of The Dresden Files: Summer Knight by Jim Butcher

Not a good time for our favorite wizard detective. His girl friend has left town to deal with an identity crisis, the Red Court of vampires wants his head, and the white council is all too eager to deliver it to them. Just when it seemed things could not get worse, the Winter Knight is murdered and the Winter Queen of Faerie comes to Dresden to figure out who-dun-it. As it turns out, you don't really have a choice when a Queen of Faerie offers you a job, you take it. No pressure though. If D...