Shameless (The House of Rohan, Book 5) by Anne Stuart
A match made in hell.
A long string of tragic loves haunts Viscount Benedick Francis Alistair Rohan. Cool and cynical, he's weary of life's fickle games and wants a prim and proper wife he can ignore while he indulges his sensual pleasures elsewhere. Lady Melisande Carstairs is nothing less than a tornado storming into Benedick's measured life.
Possessed of boundless energy and the heart of a reformer, Melisande always conquers, whether it's saving souls or seducing the man she's inconveniently fallen for. When she informs Benedick that his brother is in trouble, he's compelled to investigate, undercover. Under those covers, however, is Melisande herself, playing a dangerous game in the name of justice...
Includes the bonus story - The Wicked House of Rohan