Pop Goes the Weaselm (Alex Cross #5) by James Patterson

Detective Alex Cross is back-and he's in love. But his happiness is threatened by a series of chilling murders in Washington, D.C., murders with a pattern so twisted they leave investigators reeling. Cross's pursuit of the killer produces a suspect, a British diplomat named Geoffrey Shafer.

But proving he's the murderer becomes a potentially deadly task. As Shafer engages in a brilliant series of surprising counter moves, Alex and his fiancee become hopelessly entangled with the most memorable nemesis Cross has ever faced.