Since 1976, Robert Caro has devoted himself to The Years of Lyndon Johnson, a landmark study of the thirty-sixth president of the United States. The fifth and final volume, now underway, will presumably cover the 1964 election, the passage of the Voting Rights Act and the launch of the Great Society, the deepening of America’s involvement in Vietnam, the unrest in the cities and on college campuses, Johnson’s decision not to seek reelection, and his retirement and death—enough material, it would seem, for four ­additional ­volumes. If there is a question that annoys Caro more than “Do you like Lyndon Johnson?” it is “When will the next book be published?”

This interview took place over the course of four sessions, which were conducted in his Manhattan ­office, near Columbus Circle. The room is spartan, containing little more than a desk, a sofa, several file cabinets, and large bookcases crammed with well-thumbed volumes on figures like FDR, Al Smith, and the Kennedy ­brothers—not to mention copies of Caro’s own books. One wall is ­dominated by the large bulletin boards where he pins his outlines, which on several ­occasions he politely asked me not to read. On the desk sit his Smith-Corona Electra typewriter, a few legal pads, and the room’s only ­ornamental touch: a lamp whose base is a statuette of a charioteer driving two rearing horses.

Caro was born in New York in 1935. He was educated at Horace Mann and Princeton; after college, he worked for a New Jersey newspaper and then Newsday. It was there that Caro first heard of Robert Moses, the urban ­planner who would become the subject of The Power Broker (1974), which is not so much a biography as it is a thirteen-hundred-page examination of the political forces that shaped modern-day New York City. After conceiving of the book as a Nieman Fellow at Harvard, Caro persisted through seven difficult years of being, in his words, “plain broke.” With the support of his wife, Ina (to make ends meet, she sold their house on Long Island without telling him), he finished, and The Power Broker won Caro his first Pulitzer. It also won him the freedom to dedicate himself to his next subject, LBJ. (For his third volume, Master of the Senate [2002], he won another Pulitzer.)

In addition to the countless hours he has spent in archives poring over memos and correspondence, Caro has camped out alone in the Texas Hill Country; persuaded former senator Bill Bradley to serve as a model on the Senate floor (Bradley is roughly the same height as Johnson, making him a useful stand-in); and tracked down virtually everyone who ever knew Johnson, from his siblings to his chauffeur. Many of these sources are now ­deceased, to the frustration of Caro, who valued the ability to call Johnson aides like George Reedy or Horace Busby for spur-of-the-moment clarifications.  ...