Red Light by T. Jefferson Parker

Parker's many fans met Merci Rayborn, the Orange County homicide investigator, in The Blue Hour, and will be happy to renew their acquaintance with her in Red Light. Although she's still mourning the death of her former partner Tim Hess, who fathered her 2-year-old son, her relationship with fellow cop Mike McNally is progressing nicely, and so is her career on the force. Then two murders, decades apart, come together in a way that shakes Merci's world both personally and professionally; two beautiful young prostitutes are both killed for what they knew and what they threatened to tell. Who's covering up the corruption in the department that led to the first murder? And was Merci's lover responsible for the second? Someone's sending Merci evidence that disappeared from the police locker years ago; did that same person frame Mike too?