“Robert Caro’s The Power Broker at 50” is at the New-York Historical Society through February 2.

As I arrive at Robert Caro’s house, down a rutted, unpaved road in East Hampton, he asks me whether I’d hit any traffic on the Long Island Expressway. I had, and I remark that I’m here to talk about the man who made that happen. Caro offers a wry smile and some coffee, and even before we sit down, we get into a conversation about Robert Moses and the Long Island landscape of potato farms and old estates that his highways converted into exurbs. Caro, of course, grew famous with his first book, The Power Broker, the definitive biography of Moses and the auto-centric New York City he created through unelected iron rule. On September 16, The Power Broker will turn 50, and the New-York Historical Society is marking the anniversary with an exhibition. Even now, Caro can spin off many of the book’s revelations without looking anything up. He reminds me that, when Moses built the LIE, everyone told him to acquire an extra 40 feet of right-of-way to accommodate a light-rail line. The extra land, back then, would have cost little. “He wouldn’t do it. And he built the foundations so lightly that it could never be added.” A few generations later, Long Islanders collectively lose millions of hours to that decision every day.

When I ask him how long he’s had the East Hampton place, Caro tosses off an answer that any writer can appreciate: “1990. Second volume. The first one was the apartment in the city.” He means the payouts from the first two parts of the epic biography The Years of Lyndon Johnson, on which he’s been working since 1975. It’s an airy house with a nice deck and a sparkling pool, but we’re headed out back, down a flagstone path, to converse in the considerably more humble writer’s shack in the woods where Caro is laboring away on the fifth and final Johnson book. (His wife and ultralong-term research aide, Ina, is out for the morning, playing tennis with an old friend.) It’s an ageless space, one where it could be last week or 1950 inside, matter-of-fact and utilitarian. A couple of bookcases, a plywood work surface, corkboard with outlines tacked up, an old brass lamp, an underworked laptop for emails, a Smith-Corona typewriter. The desk chair is hard wood with no cushion. There’s a saltshaker next to the pencil cup for when Ina brings a sandwich out at midday. The desk has a big half-moon cutout, same as the one back in New York, so he can rest his weight on his forearms and ease his bad back. That arrangement was recommended by Janet Travell, the doctor who grew famous for prescribing John F. Kennedy his Boston rocker. She, with Ina, is a dedicatee of The Power Broker.

He bought the prefab shack, he says, from a place in Riverhead for $2,300, after a contractor quoted him a comically overstuffed Hamptons price to build one. “Thirty years, and it’s never leaked,” he says. This particular shed was a floor sample, bought because he wanted it delivered right away. The business’s owner demurred. “So I said the following thing, which is always the magic words with people who work: ‘I can’t lose the days.’ She gets up, sort of pads back around the corner, and I hear her calling someone … and she comes back and she says, ‘You can have it tomorrow.’”

Does he write out here every day? “Pretty much every day.” Weekends too? “Yeah.” Does he go out much while he’s on the East End? “We have two friends who live south of the highway, and I said to Ina, aside from them, I’m not going this year.” There are other writer friends nearby in Sag Harbor, and they get together, but at this age, Caro admits a little sadly, they’re thinning out. He’ll be 89 this fall.

Early this summer, Caro went to the White House. He was there not to pick up an award or meet the president but to sit in an empty room, a bit of research for the Johnson book. He deflects questions about what exactly he’s writing at the moment — “I don’t want to talk it out,” he says, because he claims the story will then lose freshness on the page — but he’ll say, at least, that he’s waist-deep in the Vietnam War, and that’s what took him to Washington. “The decisions” about the war, he tells me, “were not made in the Cabinet or the National Security Council. They were made in something called the ‘Tuesday Lunch.’” The what? He nods: “See, no one even knows. They have about 8 billion books on Vietnam, right? Every Tuesday at 1:30, he” — President Johnson — “and four guys in the family dining room on the second floor of the White House. So I was generally unhappy with the way I was handling this. I said, One of those things is you’re not feeling that room. So I had a friend who interceded for me, and I went up and sat in the family dining room while the guy they gave me, the chief usher or something in the White House, was sitting out in the hall, and he thinks I’m nuts. But you know something? It’s right next door to the master bedroom and the two girls’ bedrooms, Lynda Bird and Luci. You can hear the people out on Lafayette Avenue. So when they were eating dinner or they were sleeping, they were hearing, ‘Hey, hey, LBJ, how many kids did you kill today?’”

All that wrangling and overnight travel for one vivid paragraph, maybe a single vivid sentence. It reminds me of an earlier passage about Johnson as a young man on Capitol Hill, as he runs to work at dawn, inspired by the dome as it gleams in the sunrise. Caro got that image the same way by showing up at daybreak himself.

Once you grasp that Caro insists on chasing down every thread imaginable, long past the point where most people would shrug some things off as a case of diminishing returns, it becomes clear why these books take as long as they do. (As he has told many interviewers over the years, he’s a pretty........

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