Demon's Kiss - The Book of Demons, #1

Kidnapped, FBI Agent Angel Nicholas awakens to find herself in a vast underground chamber. Spotlighted in the center of that room is the most magnificent specimen of masculine beauty Angel has ever seen. He's absolutely gorgeous. Yet everything she's been taught tells her that he can't be what he seems to be. Yet she can't deny the evidence of her own eyes. To her astonishment, Angel finds herself instantly drawn to him. In all her life she's never reacted so strongly to a man.
Perhaps it's his pride, or his courage, in the face of his captivity, because he's just as much a prisoner as she is. Their only chance of escape lies with each other...