This is book number 7 in the Harry Bosch series.
Terry McCaleb, a retired FBI agent, is asked by the LAPD to help them investigate a series of murders that have them baffled. They are the kind of killings McCaleb specialized in solving with the FBI, and he is reluctantly drawn from his peaceful new life back into the horror and excitement of tracking down a terrifying homicidal maniac.
McCaleb's first reading of the crime scene leads him to look for a methodical killer with a taste for rituals and revenge. As his quick look accelerates into a full-sprint investigation, the two crimes his murdered loner and Bosch's movie director begin to overlap strangely. With one unsettling revelation after another, they merge, becoming one impossible, terrifying case. McCaleb believes he has unmasked the most frightening killer ever to cross his sights. But his investigation tangles with Bosch's lines, and the two men find themselves at odds in the most dangerous investigation of their lives.