Showing Kill and Tell by Linda Howard

Still reeling from her mother's recent death, Karen Whitlaw is stunned when she receives a package containing a mysterious notebook from the father she has barely seen since his return from Vietnam over twenty years ago. Karen packs the notebook away, putting it - and her father- out of her mind, until she receives a shocking phone call. Her father has been murdered.
Homicide detective Marc Chastain considers the murder nothing more than street violence, and Karen accepts his judgment - at first. But she changes her mind when she's burgled and 'accidents' begin to happen. Desperate for answers, Karen knows she needs to find that notebook again...