The Duchess Takes a Lover by Jillian Eaton

A duchess with nothing to lose…
After a disastrous wedding night followed by eighteen months of isolation at her husband’s country estate, the Duchess of Southwick has grown tired of being lonely. There’s only so many books a person can read, and Mara has read most of them twice. So she’s made a decision. For the first time since her wedding, she’s traveling to London for the Season. And while she’s there, this meek, shy mouse of a wallflower who has never met a shade of beige she didn’t like will do the unthinkable.
Mara Buxton, Duchess of Southwick, is going to take a lover.

A duke with everything to gain…
When Ambrose hears the rumors swirling about his wife, his initial reaction is scornful amusement. Mara, step out on him? Absurd. While a fiery spark of attraction first drew them together, it was extinguished long ago. The Duchess of Southwick, for all her beauty, is cold as ice, and the only thing she has a chance of inviting into her bed is snow. But when Ambrose discovers that she really does intend to have an affair, his amusement quickly turns to incredulity and then to jealousy. Mara is his wife, dammit.

If anyone is going to make her burn with passion…it’s him.