A Gentleman Ought to Know (The Duke's Estates, Bk. 4)

Charlotte Deeping needs something to keep her occupied now that she's back home after her first London season. She misses solving local intrigues with her school friends, but they've all gone off and gotten married. Then Laurence Lindley, the Marquess of Glendarvon, comes for a visit, and drops a mystery right into her lap.

When Charlotte's brother makes her promise not to interrogate the marquess, she agrees. But that only means she'll have to find subtler ways to learn about the mysterious marquess and his past. The more time they spend together, the closer Charlotte gets to finding answers, and to Laurence. That is, until Charlotte's digging rouses an old vendetta and Laurence has to delve into his own history to help the young lady he's come to love.