Werewolf cop : a novel / Andrew Klavan.

Enhanced descriptions from Syndetics:

Zach Adams is one of the best detectives in the country. Nicknamed Cowboy, he's a soft-spoken homicide detective known for his integrity and courage under fire. He serves on a federal task force that has a single mission: to hunt down Dominic Abend, a European gangster who has taken over the American underworld.After a brutal murder gives them a lead, Zach and his tough guy NYPD partner Martin Goulart are finally on Abend's trail. But things get complicated -- and very, very weird. Goulart's on-the-job enemies are accusing him of corruption. And Zach is beginning to suspect that Abend's evil goes beyond crime--perhaps to the edge of the supernatural. As his investigation continues in Germany, Zach finds himself lured into the impossible. In a centuries-old forest under a full moon, a beast assaults him, cursing him forever. In the aftermath, Zach is transformed into something horrible --something deadly.Now, the good cop has innocent blood on his hands. He has killed--and will kill again--in the form of a beast who can't be controlled or stopped. Before he can free himself, he's going to have to solve the greatest mystery of all: How can you defeat evil when the evil is inside you?