Buried Too Deep by Karen Rose

Three can keep a secret if two of them are dead.

Employed as the nighttime security guard of Broussard Investigations, Phineas Bishop has been working through overwhelming PTSD episodes from his army service while still utilizing his military skills. But when a violent break-in occurs at the office, the accusatory eyes of the NOPD are on Phin, and he resolves to track down the intruder and clear his name.
Phin’s only lead is Cora Winslow, a spirited librarian who also needs answers. The body of her father, murdered twenty-three years ago, has just been discovered under a recently demolished building. So who has been sending her handwritten letters—written and signed by her father—every year since she was five? Someone wants to keep Cora in the dark. And now, they’re coming for her.
As Cora’s self-appointed bodyguard, Phin is surprised by his growing fondness for the woman and her fierce determination and research prowess. But New Orleans’s Garden District holds secrets as old as the streets themselves. With help from the entire Broussard P.I. team, Phin and Cora enter a labyrinth of fraud and homicide that threatens to bury them all.