One True Love (A Regency Romance) by Lisa Follett
Lord William Prescott is in want of a wife. Unfortunately, the lady he wants is betrothed.
Miss Cassandra Chambers simply cannot understand her attraction to Lord William. After all, she is supposed to be in love with Mr. Miles Parker. In a scandalous twist of fate, Cassie's future changes when Lord William rides to her rescue, saves her from ruin, and makes her his wife.
When William comes to believe Cassie's heart may always belong to another, he is determined to lock away his own. If only he could lock away his desire for his One True Love.