A Gentleman Ought to Know (The Duke's Estates #4)
Jane Ashford

When Charlotte's brother makes her promise not to interrogate the marquess, she agrees. But that only means she'll have to find more subtle 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.