Small Favour - The Dresden Files, Book Ten

Meet Harry Dresden, Chicago's first (and only) Wizard P. I. Turns out the 'everyday' world is full of strange and magical things - and most of them don't play well with humans. That's where Harry comes in. 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 removing stains his apprentice bungled into his carpeting. The future seems bright.
Unfortunately, the past isn't looking 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 her two favours, and it's time to pay up. It's a small favour he really can't refuse, but it will trap Harry between a nightmarish foe and a deadly ally, stretching his skills and loyalties to their limits. It figures.
Everything was going too well to last. Magic - it can get a guy killed.