The Sweet Dove Died by Barbara Pym

A chance encounter over a Victorian flower book brings together Humphrey, an antiques dealer, his nephew James, and Leonora. Although she is considerably older, Leonora becomes a little too fond of James. Leonora is determined to keep James to herself but realizes that her rival is the bookish Phoebe. Then along comes Ned, a saucy young American, and Leonora's problems boil over.