Triptych (Will Trent #1) by Karin Slaughter

In the city of Atlanta, women are dying at the hands of a killer who signs his work with a single, chilling act of mutilation. Leaving behind enough evidence to fuel a frenzied police hunt, this cunning madman is bringing together dozens of lives, crossing the boundaries of wealth and race. And the people who are chasing him must cross those boundaries too. Among them is Michael Ormewood, a veteran detective whose marriage is hanging by a thread and whose arrogance and explosive temper are threatening his career. And Angie Polaski, a beautiful vice cop who was once Michael's lover before she became his enemy.

But another player has entered the game: a loser ex-con who has stumbled upon the killer's trail in the most coincidental of ways someone who may be the key to breaking the case wide open...