Small Favor (The Dresden Files #10) by Jim Butcher

The New York Times Bestseller

Harry Dresden's life finally seems to be calming down -- until a shadow from the past returns. Mab, monarch of the Sidhe Winter Court, calls in an old favor from Harry -- one small favor that will trap him between a nightmarish foe and an equally deadly ally, and that will strain his skills -- and loyalties -- to their very limits.