Ghost Moon by Rebecca York

A freed slave from a parallel universe, Quinn has come to this world on a vital mission. But then she's stopped in her tracks by the seductive attentions of a mysterious spirit. He's a ghost named Caleb Marshall-a werewolf who was supposedly murdered by the ancestors of her dearest friends.

When Caleb finds a way to take over a human body, Quinn can no longer resist her overwhelming desire, even though she's not sure she can trust him. But soon she discovers that they have a shared enemy-and if she doesn't act fast, she may lose the man she's come to love, along with her life.