Plenilune by Jennifer Freitag

The fate of Plenilune hangs on the election of the Overlord, for which Rupert de la Mare and his brother are the only contenders, but when Rupert’s unwilling bride-to-be uncovers his plot to murder his brother, the conflict explodes into civil war.

To assure the minds of the lord-electors of Plenilune that he has some capacity for humanity, Rupert de la Mare has been asked to woo and win a lady before he can become the Overlord, and he will do it—even if he has to kidnap her.

En route to Naples to catch a suitor, Margaret Coventry was not expecting a suitor to catch her.