Wicked Nights by Gena Showalter — book slip

Leader of the most powerful army in the heavens, Zacharel is on the brink of losing his wings due to his ruthless nature. His icy heart is unyielding until he encounters Annabelle Miller, a human accused of a crime she didn't commit.

Annabelle has spent four years in an institution for the criminally insane, haunted by demons and pursued by their king.

Zacharel becomes her only hope for survival, but as their worlds collide, Annabelle must decide if the brutal angel is her salvation or her ultimate damnation.

First in the Angels of the Dark series.