A duke of her own / Eloisa James.

Enhanced descriptions from Syndetics:

A duke must choose wisely . . .

Leopold Dautry, the notorious Duke of Villiers, must wed quickly and nobly--and his choices, alas, are few. The Duke of Montague's daughter, Eleanor, is exquisitely beautiful and fiercely intelligent. Villiers betroths himself to her without further ado.

After all, no other woman really qualifies. Lisette, the outspoken daughter of the Duke of Gilner, cares nothing for clothing or decorum. She's engaged to another man, and doesn't give a fig for status or title. Half the ton believes Lisette mad--and Villiers is inclined to agree.

Torn between logic and passion, between intelligence and imagination, Villiers finds himself drawn to the very edge of impropriety. But it is not until he's in a duel to the death, fighting for the reputation of the woman he loves, that Villiers finally realizes that the greatest risk may not be in the dueling field . . .

But in the bedroom. And the heart.