Jim Butcher's The Dresden Files: Fool Moon #3

Chicago is awash in the blood of a werewolf's rampage -- and Harry Dresden is caught right in the middle of it. As he desperately races against the rise of the next full moon, Harry is faced with not one, but two faustian bargains -- one of them offering him the tantalizing truth behind his mother's life and death. Before the full moon sets, Harry will have born the loss of one friend through violence, the other through betrayal...