The Executor by Jonathan Kellerman

In this gripping psychological thriller, a young psychologist named Dr. Jeremy Carrier finds himself entangled in a web of mystery and danger following the brutal murder of his girlfriend. As he grapples with his grief, Jeremy is drawn into a clandestine group of powerful individuals who offer him cryptic clues and unsettling insights into a series of unsolved crimes. With his professional life unraveling and his personal safety at risk, Jeremy must navigate a treacherous path of deception and betrayal to uncover the truth, all while confronting his own inner demons and the dark secrets that threaten to consume him.