Convenient Marriage By Heyer

This is one of Heyer's earlier novels, which has a similar plot to April Lady. In both novels a mature sophisticated man marries a naive girl which looks like a marriage of convenience. Marcus, the earl of Rule, is expected to marry Elizabeth Winwood, sister of a rakish gambling peer. Elizabeth however does not want to marry him, she is in love with Edward Heron, a younger son of a local family who is a soldier. Horatia, the younger sister of Elizabeth, goes to Lord Rule and suggests that he marries her instead. Rule is startled but he agrees to the marriage and tells Horatia that he will help Edward in his career. Rule has a former mistress, Caroline Massey, who engages in plotting with Robert Lethbridge to break up the marriage and force a divorce between him and Horatia. Robert eloped with Louisa, Rule's sister and although he rescued her, the 2 men have been enemies. The book has a complex plot where Rule tries to manage his teenaged bride, but without being too strict, and she insists on pursuing a friendship with Lethbridge. Eventually, Rule wins his wife's heart and tells her that he has broken with his mistress and they will be a happy couple.