The Duke Is a Devil by Karen Lingefelt

Dane Armstrong, Duke of Bradbury, has caused enough trouble for aspiring author Cecily Logan that she's written a book about him—but for her eyes only, since it reads as if she might be in love and hoping to trap him into marriage. When the manuscript is stolen by a third party who schemes to profit from it unless the duke yields to blackmail, Cecily courts scandal by calling on Bradbury to ply his so-called ducal powers against publication—or one of them will face ruin and ridicule.

What Cecily doesn't know is that Dane's already used those fabled powers to read the book and learn the secret she fears will make her a laughingstock. The devil in him wants to see how far she'll go to prevent the disaster she imagines, even to plotting seduction and surrendering to wicked temptation. Cecily may not have to forfeit her soul, but Dane soon finds himself in danger of losing his heart.