The unforgiven
(Book)

Series numeration from goodreads.com.

Twelve years after the grisly murder of her parents, Kaitlyn Delaney has finally found peace. She has friends, a good job, a place to call home and a new life to live. But then a shadow creeps in from Katie's past, reminding her that she will never completely escape its terrifying grip. When private investigator Dan Oliver is called to the scene of a gruesome crime in New Orleans, he can't help but hear echoes of the Delaney case, the unsolved murder that made him leave law enforcement. As he digs deeper, he unearths more chilling similarities--including mysterious letters connecting the killer to a string of murders that terrorized the Big Easy in 1919. Now reunited after all this time, Dan and Katie scour the streets together, desperate to find answers before more lives are lost. But the otherworldly roots of this evil run far deeper than they ever imagined ... and only the most precious of sacrifices will bring its twisted reign to an end.