A Dance in Moonlight (The Fitzhugh Trilogy)

After losing her childhood sweetheart to another woman, Isabelle Englewood is heartsick. But then something remarkable happens: Upon arriving at Doyle's Grange, her new home, she meets Ralston Fitzwilliam, who looks almost exactly like the man she cannot have. Come late at night, she tells him, so I can make love to you pretending that you are the one I love. Little does she realize what she is about to unleash.