Daddy's Little Promise. Daddy Series, #21

Emily's life has knocked her down so many times she's surprised that she's still alive. She thinks her life can't get any worse until she falls into the hands of a human smuggler, and he plans to keep her as his slave. She thinks this might be what finally kills her. Grey promised his friend, Darian, that he'd do his best to get Emily out of the bastard's clutches. His first look at the thin, pale young woman made his fury rise up.
It took everything in him to keep from killing the guy. The most important thing was to get her out of there. He promised to take care of her and keep her safe, and when he fails, he's terrified she'll never trust him again.