A Marquess for Convenience by Bianca Blythe

Lord Arthur Carmichael, Marquess of Bancroft, is at a crossroads. His wild reputation was the perfect cover for the spy missions he undertook for the Crown. Now, that reputation could keep him from settling into a cabinet position–unless he takes a wife.
Before he can embark on the dreaded wedding quest, he must complete one last mission: catch a thief who’s stolen four of five priceless French jewels. If the last one disappears, the fragile peace between France and England could be damaged beyond repair.

Whom should he encounter on the French Riviera but Lady Mulbourne, who never forgave him for vanishing during their long-ago courtship. Arthur suspects her nimble fingers are adept at more than just fluttering a fan, because he knows of one item she’s already stolen: his heart.