Monster by Jonathan Kellerman is a tight but reasonably complex psychological thriller / whodunnit that sticks to the case, no distracting subplots.
A detective and his psychology consultant investigate two similar grisly murders with one victim the employee of a secure institution for the criminally insane. Is there a connection? Their investigations turns up more possibly related murders in the past and present, and an inmate of that institution who couldn’t possibly be involved in the recent crimes but seems to know something.
Fascinating, suspenseful, one of best of it’s kind I’ve read.