1st to Die (Women's Murder Club #1) by James Patterson

Each one holds a piece of the puzzle: Lindsay Boxer is a homicide inspector in the San Francisco Police Department, Claire Washburn is a medical examiner, Jill Bernhardt is an assistant D.A., and Cindy Thomas just started working the crime desk of the San Francisco Chronicle.

But the usual procedures aren't bringing them any closer to stopping the killings. So these women form a Women's Murder Club to collaborate outside the box and pursue the case by sidestepping their bosses and giving each other a hand. The four women develop intense bonds as they pursue a killer whose crimes have stunned an entire city. Working together, they track down the most terrifying and unexpected killer they have ever encountered--before a shocking conclusion in which everything they knew turns out to be devastatingly wrong.