As “The Diamond as Big as the Ritz” opens, sixteen-year-old John T. Unger is leaving the small middle-class town of Hades to attend St. Midas School near Boston, “the most expensive and the most exclusive boys’ preparatory school in the world.” His mother packs his trunk, his father gives him money, and after a tearful goodbye, John T. Unger is off to attend school with boys from the country’s wealthiest families.

In his second year at St. Midas, John meets Percy Washington. Well-dressed and reserved, Percy has little to say about his home or family, until he invites John to spend the summer at his family’s home in Montana. On the train ride to Percy’s home, Percy tells John that his father is “the richest man in the world” and that he...