Thief of Dreams by Mary Balogh

On the eve of her twenty-first birthday ball, Cassandra would finally be free. Free from mourning her father’s death, and free to begin a new life as Countess of Worthing. Cass had no intention of marrying — not yet. Instead, she planned to rule her land and bask in her independence.
Then the dashing Nigel Wetherby arrives at the ball — an uninvited guest on Cassandra’s most magical day. To young Cass, he is the sweetest of passions. But Nigel has plans of his own. Plans that could break her heart, and defy their love. At once, he is both her worst nightmare — and her dream come true …