EVEN IN THE DAKOTA, the storied apartment building on New York’s Upper West Side, home to such cultural icons as Yoko Ono and Lauren Bacall, Buddy Fletcher stood out. African American, fantastically accomplished, and wealthy, Fletcher had grown up in modest circumstances, gone to Harvard, and graduated as “first marshal” of his class in 1987. From there he hit Wall Street, earned millions before he turned 25, and started his own firm, Fletcher Asset Management. By the time he was 30, the company was operating as a hedge fund and boasting of triple-digit returns. Fletcher made multimillion-dollar donations to Harvard,...