The American Cousin by Dawn Lindsey

Miss Sorrel Kent told herself that she felt no emotion except antipathy for the Marquis of Wycherly. He was an English lord, while she was an American who scorned all titles. He was as arrogantly dominating as she was fiercely independent. And above all, he was the intended husband of her ravishing cousin Livia, who could have any man she wanted and who most certainly wanted the dazzlingly handsome marquis. 
Sorrel had come to England to recover from one lost love - she did not seek another heartbreak. But her best intentions turned into her worst fears when the sparks that flew between them suddenly, startlingly, and shockingly ignited an unthinkable passion's fire....