Not Quite A Lady. Number 4 in series

Darius Carrington is a spectacularly handsome rake with a rare intelligence and no heart. A man of science, he divides his time between bedding loose-moralled women and writing scholarly papers about livestock breeding. He finds the eligible ladies of the ton thoroughly dull. That is until he meets Lady Charlotte Hayward... Lady Charlotte is so beautiful, charming, and gracious that no one has noticed what an expert she is at Not Getting Married.
Early on, she learned a hard and painful lesson about trust... and temptation. In the years since, she's built impentrable defences that no man has overcome. She's devoted to life to being everything she ought to be- and she's not about to let a man like Carsington entice her to anything she shouldn't. But the rules of attraction can easily overpower the rules of manners and morals, and sometimes even the best behaved girl has to follow her instincts, even if it does mean risking everything.