Monster (Alex Delaware #13) by Jonathan Kellerman

A corpse is found in a car trunk. The victim was a twenty-five-year-old would-be actor called Richard Dada. He had been sawn in half. Eight months later, the body of Claire Argent, a psychologist at a hospital for the criminally insane, is discovered. She was mutilated in the same horrific way.

Detective Milo Sturgis is put on the case and, when the incoherent ramblings of a patient locked up in a mental hospital for the criminally insane begin to make terrifying sense, he calls on psychologist Alex Delaware to help him delve into the muddy waters of insanity.

Can Alex and Milo unravel this dark web of family secrets, vengeance and manipulation in time to stop further killing?