Seducing an Angel — Mary Balogh

He is to be wealthy, wellborn, and want her more than he wants any other woman. Those are the conditions that must be met by the man Cassandra Belmont will choose as her lover. Marriage is out of the question for the scandalous widow who must now barter her beauty in order to survive. With seduction in mind, she sets her sights on Stephen Huxtable, the irresistibly attractive Earl of Merton and London's most eligible bachelor. But a single night of passion alters all the rules. Cassandra, whose reputation is already in tatters, is now in danger of losing the one thing she vowed never to give. And Stephen won't rest until Cassandra has surrendered everything -- not as his mistress, but as his lover and his wife.