The Ripper Letter by Katherine Ramsland.
Book One of The Hearts of Darkness Series.
When a murdered historian is marked with a mysterious code, homicide detective Dee Brentano worries about his colleague – her missing father, Alexandre. FBI special agent J. R. Pierce tells her that Alexandre is wanted for this murder.
Desperate to find him first, she discovers that Alexandre has items that several people – including Pierce – would kill to possess. One is a letter attributed to Jack the Ripper. Another is an erotic cryptograph.
Dee encounters a potential ally in Detective Gregory Brenner. She’s attracted to him, but fears that he’s playing her to find her father. She’s also drawn to her father’s protégé, Scott Bateman, who can decode the Ripper letter’s secret message and the symbol on the murdered historian. It’s bait for luring supernatural entities. It’s also a map to locating her father.
Dee must choose her path wisely. One leads to a supernatural lover, the other to an immortal serial killer.
A good read with likeable characters. Took a while to read. 3*.

I love Catherine Ramsland! This was another incredibly interesting and intriguing read from her. The characters were very engaging and there were some really great, unexpected elements to this story.