Motive (Alex Delaware #30) by Jonathan Kellerman

Even hundreds of closed cases to his credit can't keep LAPD police lieutenant Milo Sturgis from agonising over the crimes that don't get solved - and the victims who go without justice. Victims like Katherine Hennepin, a young woman strangled and stabbed in her home. A single suspect with a solid alibi leads to a dead end - one even psychologist Alex Delaware's expert insight can't explain.

The only thing to do is move on to the next murder case - because there is always a next one.

This time the victim is Ursula Corey, a successful, attractive divorcee who's been gunned down - not a robbery but an execution, a crime that smacks of simple, savage revenge. And along with that theoretical motive come two strong contenders for the role of suspect: the dead woman's business partner/ex-husband and her divorce lawyer/secret lover. But just as Alex and Milo think they're zeroing in on the killer, a bizarre new clue stirs up echoes of the unsolved Hennepin murder. And the discovery of yet another crime scene bearing the same taunting signature raises the spectre of a serial killer on a mission, whose twisted method is exceeded only by his manipulative and cunning madness.