«The Duke and I» by Julia Quinn

Simon Basset, the irresistible Duke of Hastings, has hatched a plan to keep himself free from marriage: he will pretend to be engaged to the lovely Daphne Bridgerton. After all, it isn't as if the brooding rogue has any real plans to marry… Although, there is something about the alluring Miss Bridgerton that sets Simon's heart beating a bit faster. But Daphne soon finds herself forgetting that their courtship is a complete sham…