Extremity — A.K. Lawrence

Detective Sean Stone is visiting his hometown for a vacation before he begins his new job as an agent of the FBI. He's called in to a grisly murder in the Student Ghetto of Kalamazoo, Michigan, and he takes his partner and best friend Nino with him. The murder sparks a trail of mayhem that spans the country.
Each murder finds a photographic journal of the women's unpleasant end and a letter addressed to Agent Stone. The killer seems to know everything about him and Agent Stone chases his tail in his desire to capture and put this sick man behind bars.