Undercover Hunter (Harlequin Romantic Suspense Series #1831)

In this story from New York Times bestselling author Rachel Lee, two investigators must learn to play nice before it's too late 

Detective Cade Bankston never had any luck with female partners. So when he's assigned to work with feisty, raven-haired DeeJay Dawkins, he isn't pleased at all. Posing as a married couple, the investigators must team up to catch a killer. That is, if they don't kill each other first. 

Putting their mission first proves tricky as mutual disdain evolves into mutual desire. But distraction is not an option. The killer who seemingly vanished five years prior has returned to Conard County, Wyoming, to finish what he started. And he just set his sights on two new victims.