Her Summer Duke by Arietta Richmond

A Lady promised since childhood, now questioning her feelings, a Duke also promised, and questioning his desire for love, a nobleman turned to the sea, now desperately in love, a cousin doubting her own betrothal, a confusion of suitors, an unexpected triumph of love.
Lady Margaret Caulfield is somewhat spoiled, and newly come out into society, finds that she suddenly doubts everything – about love, and about her future. Promised since childhood to the man who saved her life when she was ten, she had not questioned it – until she met a man at a Masquerade Ball, and immediately fell in love with him.
Captain Lord Adam Halstone is barely accepted by society, and only then because he is the second son of a Duke – for he has done the inexcusable, and not only invested in trade, but chosen to work as a ship’s Captain. Love is not something that he has considered, yet when he meets a green eyed beauty at a Masquerade Ball, he finds himself utterly in love.
But Lady Margaret must contend with her uncertain feelings for the Duke she is promised to, who himself is coming to doubt the wisdom of the promise he made eight years before. And matters are complicated by others who would also seek Lady Margaret’s hand, should she cry off from that promise.
In the end, will love triumph? Or will duty bind each of the players in this tangle to an unhappy future?