The Duchess by Jude Deveraux

The story starts in 19th-century Scotland, where Claire Willoughby, an American heiress, falls in love with mama's boy Harry Montgomery largely because he is blond and a titled duke. Harry wants Claire because she will inherit ten million dollars as soon as she weds. Harry whisks the bookish Claire, along with her childish parents and precocious sister ``Brat,'' to his Scottish castle, Bramley, where an odd bunch of family members roam in and out of dark passageways mumbling about horses and dogs.

In this chilly atmosphere, Claire stumbles upon the cynical but sexy Trevelyan, who, she later learns, is her childhood hero, explorer Frank Baker, whose writings about the exotic and erotic have held her in a state of dire excitation all these years. Passion bubbles...

Finally, the highly emotional heiress grapples with a moral dilemma as it dawns on her that although she loves the lecherous Trevelyan, she must marry the boring Harry in order to secure her inheritance. The final twist reveals the real identity of Trevelyan/Baker, and leaves Harry right where he belongs...