Gift of Love by Cynthia Moore

Since losing her intended at the Battle of Waterloo, Lady Rebecca Hastings has lost her joy for living. On what would have been her wedding day, she is reunited with a childhood friend and neighbor, Adrian Russell, Marquess of Burton. As they reminisce about their childhood together, she is immediately conscious of new feelings for him. The cheer of the Christmas season and some puzzling visits from the spirit of her intended eventually help Rebecca realize her enduring love for Adrian. But can he ever accept her as more than a friend?