The Forbidden Lord by Sabrina Jeffries

Jordan Willis, the Earl of Blackmore, has played with the heart of many a society belle—yet one stolen kiss impulsively planted on the soft, irresistible lips of the preacher's daughter proves his undoing. The prim and proper Emily Fairchild's innocent response sends desire surging through the notorious rake, and he can't forget her. Now, meeting Lady Emma Campbell, an outrageously flirtatious debutante, leaves the earl more shaken than ever—for the brazen beauty is an uncanny double of the sweet Emily!Emily cannot reveal the reason for her scandalous masquerade—not even to the earl whose heated embrace awakens a sizzling passion. Her very life depends on convincing society that she is Lady Campbell, yet no disguise can hide her dangerous attraction to the earl. But does "Cinderella' dare risk losing her handsome "prince" by revealing that she is just a country girl?