From Dusk to Dawn: Nightshade / Night Smoke book by Nora Roberts

Danger, secrets and romance lurk under the cover of darkness…

Nightshade

Lieutenant Althea Grayson does everything by the book. So when her informant on a runaway case is murdered, Althea turns to the next logical contact—Colt Nightshade, the witness at the crime scene. Colt might be a close friend of Althea's former partner, but his renegade style of justice is of no use to her. Accustomed to working on his own, Colt has to adjust to being on a team…and he can't help but feel a bit distracted by the smart lieutenant. Growing attraction between the unlikely pair complicates an already troublesome case, but Colt's never been one to turn down a challenge—and neither has Althea.

Night Smoke

When her warehouse is destroyed in a suspicious fire, businesswoman Natalie Fletcher joins forces with arson investigator Ryan Piasecki to solve the case. Ryan is as known for his shrewd detective work as his infuriating arrogance, though neither impresses Natalie the way Ryan hoped they would. Ryan is used to proving himself, though, and he knows he'll eventually be able to win Natalie over. But when he finds out that someone is purposely trying to burn Natalie out of business, the case suddenly becomes much more personal…