Under The Marshal's Protection by Anna J. Stewart

He wants to protect her...from herself.

US Marshall Howell McKenna has every reason to mistrust federal prosecutor Kara Gallagher. He’s also dedicated to his job. When she asks for his help convicting a mobster, he agrees — because it’s the right thing to do, and because her emotional attachment to the case means she’s in danger. Working together, they discover that putting their past behind them is easier than ignoring the heat that simmers between them. And, as an already menacing case takes unexpected twists, Howell’s not always sure that Kara wants justice more than she wants revenge...