Small Favour: The Dresden Files, Book Ten by Jim Butcher
Harry Dresden is feeling happy. No one's tried to kill him in nearly a year, and the worst problem he's had lately is trying to remove the stains his apprentice bungled into his carpeting. The future looks fairly bright. Unfortunately, the past isn't nearly so optimistic. An old bargain placed Harry in debt to Mab, faerie monarch of the Winter Court and the Queen of Air and Darkness. Harry still owes the Winter Queen two favours, and it's time to pay one of them off. It's a small favour that he really can't refuse, but it will trap Harry between a nightmarish foe and an equally deadly ally, stretching his skills and loyalties to their very limits. It figures. Everything was going too well to last.