Cross & Crown by Abigail Roux

When Nick O’Flaherty arrives at the scene of a double homicide to find he has a witness to the crime, he thinks it’s his lucky day. But when he realizes his witness is suffering from amnesia and can’t even remember his own name, Nick wishes he’d gone with his gut and put in for vacation time.

Then Nick’s boyfriend and former Recon teammate, Kelly Abbott, joins him in Boston, and Nick finds his hands a little too full as the case and his personal life collide. The witness he’s dubbed “JD” is being tailed by Julian Cross, a retired CIA hitman. To complicate matters further, JD forms an attachment to Nick that Nick struggles not to respond to as they search for the key to JD’s identity.

Trying to determine whether JD is friend or foe as they investigate the crime puts them on the trail of a much older mystery. When multiple attempts are made on their lives, Nick is forced to turn to old enemies and new allies to solve a centuries-old crime before he and Kelly get added to the history books.