"Mr. Cavendish, I Presume" by Julia Quinn
It's true. He doesn't. Thomas rather likes having a fiancée—all the better to keep the husband-hunters at bay—and he does intend to marry her . . . eventually. But just when he begins to realize that his bride might be something more than convenient, Thomas's world is rocked by the arrival of his long-lost cousin, who may or may not be the true Duke of Wyndham. And if Thomas is not the duke, then he's not engaged to Amelia. Which is the cruelest joke of all, because this arrogant and illustrious duke has made the mistake of falling in love . . . with his own fiancée!"
Amelia was a wonderfully thought out heroine, who deserved her own book. She was smart and a bit wicked, but she played off of Thomas, who was stiff and dull, and who's sense of duty seemingly squashed all the life out of him. Quinn's characters typically are consumed with passion for one another, but these two were stifled in that aspect. There was little romance between them. It wasn't until the 80 or so pages that these two really came together and it just felt like too little, too late.
All in all, a romantic story, but one that could easily have been included in her first book, "The Lost Duke of Wyndham".