Broken (Book 1, The Watcher Chronicles) by S. J. West

As an agent for the Watcher Agency, Jess Riley is used to dealing with intense situations. When she’s recruited into a task force headed by her boss, she didn’t plan on falling in love with a man left scarred and broken by his past sins. Jess quickly learns that fighting demons is a lot easier when you have an angel you love fighting by your side.