A secret affair / Mary Balogh.

Enhanced descriptions from Syndetics:

Beloved New York Times bestselling author Mary Balogh has written her most beguiling novel yet, in which the black sheep of the scandalous Huxtable family finally meets his match in a woman of even more wicked reputation. Her name is Hannah Reid. Born a commoner, she has been Duchess of Dunbarton ever since she was nineteen years old, the wife of an elderly duke to whom she has been rumored to be consistently and flagrantly unfaithful. Now the old duke is dead and, more womanly and beautiful than ever at thirty, Hannah has her freedom at last. And she knows just what she wants to do with it. To the shock of a conventional friend, she announces her intention to take a lover and not just any lover, but the most dangerous and delicious man in all of upper-class England: Constantine Huxtable. Constantine's illegitimacy has denied him the title of Earl, so now he denies himself nothing . . . or so the ton would have it. Rumored to be living the free and easy life of a sensualist in his country estate, he always chooses recent widows for his short-lived affairs. Hannah will fit the bill nicely. But once these two passionate and scandalous figures find each other, they discover that it isn't so easy to extricate oneself from the fires of desire'without getting singed. For the duchess and the dark lord each have startling secrets to reveal, and when all is said and done, neither will be able to say which one fell in love first, who tamed whom, and who has emerged from this game of hearts with the stronger hand.