The Rational Faculty. Hazard and Somerset: A Union of Swords, #1

Three months have passed since Emery Hazard and John-Henry Somerset faced a madman and lived to tell about it. Three months have passed since Emery Hazard resigned from his job as a detective. Three months can be too long and too short, all at the same time. On Halloween, a professor at the local college is murdered in his apartment, in front of dozens of witnesses. Then the killer disappears. Somers is assigned the case-and a new partner.
While Somers investigates the murder, Hazard struggles to find purpose in his new freedom. Despite his decision to stay away, he finds himself drawn to the case. But he's no longer police, and in the small town of Wahredua, not all of his former colleagues are happy to see him investigating another crime. When the sheriff's son and husband go missing, though, the case becomes more complicated than either Hazard or Somers had expected.
And soon they learn that someone else is manipulating events in Wahredua. Someone who is very interested in Emery Hazard. The Rational Faculty is the first book in a follow-up series; read Hazard and Somerset's first adventures in The Hazard and Somerset Mysteries, beginning with Pretty Pretty Boys.