Jim Butcher's The Dresden Files: Fool Moon #2

Harry's been on the outs with his close friend and most important client, Lt. Karrin Murphy, for months. Now, she's finally brought him back into the fold -- to help her solve a series of brutal, bloody murders that appear to have been committed by a werewolf. But even as he begins his investigation, Harry's problems are already piling up: there's no shortage of suspects, and apparently, no shortage of werewolves in Chicago; the Chicago PD still looks at him with suspicion; and the FBI doesn't trust him or Murphy. And his old "friend", mob boss Gentleman Johnny Marcone, is waiting for the right moment to enter the fray...