Devil in the Details by Madison Chase (Road to Hell Series #2)

Fresh from her own attempted murder, Charlie is ready to confront the demon who started it all--and her attraction to him. Unfortunately, an ill-timed disappearance cuts her interrogation short. Normally she'd be happy to put her detective hat on, but having to work alongside the man who divorced her and nearly got her killed makes for a hellishly hostile work environment.

Lucifer has never been more confused, intrigued, or aroused by a mortal as he is with Charlie Brant. She had seemed more receptive following her little brush with death, but now she was cold, cut-off, and much too eager to spend time with her ex-husband. The King of Hell decides to get some answers of his own and travels the road leading to the center of Hell to get them straight from the source.