Tonight and always - Nora Roberts

Kasey was an anthropologist, but her knowledge of men in general hadn't prepared her for one man in particular: Jordan Taylor.
She was to help him with the research for his novel, but the study of the human heart soon took up all their time. Sensation was their textbook and passion their teacher as they searched for the meaning of love and tested the strength of their desire, day by day--and night by night.