Jeffries, Sabrina. Duke's men.

"Maximilian Cale, the Duke of Lyons, long ago buried his grief for his missing elder brother, Peter, who was presumed dead after being kidnapped. When a mysterious note arrives from Tristan Bonnaud asserting that the Duke's brother is alive, it leads Max straight to the winsome Lisette Bonnaud, illegitimate daughter of a viscount and Tristan's sister. Soon he and Lisette are traveling to Paris posing as husband and wife, in search of Tristan, who has disappeared. And the longer he spends with Lisette, the easier it is for Max to see that the line between dukedom and desire is easier to cross than he imagined."--from publisher's description