The historian Robert Caro says he has built his career exploring a single grand theme: “how political power really worked.” Not how it worked in theory, or how it was supposed to work. His subjects were two men who achieved and wielded power like few others: Robert Moses and Lyndon Johnson. Moses built Lincoln Center, the West Side Highway, the F.D.R. Drive, the Verrazzano-Narrows Bridge and many other vital pieces of the New York landscape; Johnson built the Great Society and led the U.S. into war in Vietnam. Thanks to his Moses biography “The Power Broker” and his mammoth life of Johnson, whose fifth and final volume he is struggling to complete, Mr. Caro is one of the most revered historians living.