When the widow of a friend (Terry McCaleb) calls Bosch to investigate her late husband's mysterious death, he is there.
Robert Backus: former FBI Behavioral Science, former mentor, former Poet. Is he back? All signs point in that direction, and Agent Rachel Walling is called out of exile in the badlands of North Dakota...but she is not given any power to actually investigate what appears to be his work.
Meanwhile, during the investigation of the death of McCaleb, Harry comes across cryptic noted and pictures of McCaleb's family without their knowledge. Was McCaleb being targeted? By who? Why? One of the pictures is taken in Nevada of a road sign, ZZyzx. Bosch decides to go and see what it leads to.
Walling and Bosch, both pretty much outsiders, he is former LAPD, retired, and she is not in good standing, They are paired because Agent Walling is already there and he has information. Are the two cases a combination of the work of one madman, or was this all a coincidence. Bosch doesn't believe in coincidences. There are hints as to whom is doing the killing, but why, and how were they lured to their excruciating deaths.
Then, Harry's ex wife is keeping a secret. He knows she is and this is one of those mysteries he must solve. However, when he discovers what this life changing secret is, he could never have imagined what it turned out to be.
The book was interesting how the crimes were solved, they were led to certain places that apparently they were led to intentionally. They were being played.