In his new book, Working, the great American biographer Robert A Caro devotes an entire chapter to the importance of a sense of place, something he believes to be every bit as vital in the writing of history as in fiction. His readers must, he says, be able clearly to visualise all that is being described, to the degree that they may feel almost as if they were there at the time; only then will they understand the wider, more resonant points the author seeks to make. So let me first set the scene for my encounter with Caro, which takes place in New York on a cold, crystalline spring morning, when the newspapers are once again full of Mueller, Barr and Trump, and the cable channels are powering through their furious, frenetic cycles more frantically than ever.

Caro used to work in a small, almost monastic-sounding office at Columbus Circle. Not long ago, however, his landlord decided to sell, and he moved to premises close to Central Park on the Upper West Side, a full apartment this time. It is in an elegantly old-fashioned but rather anonymous building: a collection of small, airless landings surrounded by faceless doors that brings to mind Raymond Chandler; if the elevator opened to reveal an unshaven man in a trench coat and trilby hat, you would hardly be surprised. But in fact it opens, on this occasion, to reveal Caro himself, neat in his customary suit and tie, who is running late after a visit to the New York Times (after the recording of a podcast, he was invited, to his great delight, to attend the paper’s editorial meeting).

He unlocks the door. Inside, there is a sitting room, rarely used, a galley kitchen, and the room in which he works, where I’m to wait while he makes coffee. I half wonder if this is a test, for it takes every bit of my willpower not to snoop while I’m alone. Caro, who is now 83, has been working on his acclaimed five-volume biography of Lyndon B Johnson since the mid-70s; the fourth volume, The Passage of Power, came out in 2012 and now the world waits, somewhat anxiously, for the final book. Will he ever finish it? And if so, what revelations will it contain? Caro never talks about his work in progress, something he insists goes back to his days as a reporter (once he’d finished pitching a story to his editor, he often found he no longer had any interest in writing it), but which I’d attribute more to writerly superstition. But either way, from my seat at one side of his huge desk I can see the outline for the book pinned, sheet after sheet of it, on a cork board, and right under my nose are trays containing photocopied papers from – you can’t miss the stamp – the LBJ Presidential Library in Austin, Texas. My hands, shoved deep in my pockets, twitch slightly. I am so tempted to take… just… one… lo…

In he comes, bearing two mugs. He looks chary, and I know perfectly well why. Caro hates to be interrupted during the working day. He turns off the phone. He doesn’t have email. Once, he was typing away in the Frederick Lewis Allen Room at the New York Public Library when he was tapped on the shoulder by someone wanting to go to lunch with him – at which point, he found himself on his feet with his fist drawn back to punch the guy, an elderly, mild-mannered chap who was cringing before him. Only in the nick of time did he “get a grip” and apologise. For at least an hour after he gets home, it’s best to give him a wide berth – or so his wife Ina thinks. But he’s also one of those enthusiasts who in the end can’t help himself: reticent though he may be, the intense interest he feels for his work in general, and for LBJ in particular, rises inside him like steam and, eventually, out it comes.

There is general amazement that the famously tortoise-like Caro last year broke off from LBJ in order to produce a short, snappy book – Working – about how he writes. Wasn’t it a wrench? Why on earth would he waste his time? But he doesn’t see it like this. “I didn’t feel anxious about it,” he says, in his wonderful voice (he has the kind of New York accent you don’t hear so often any more: “toime” for time, “cawfee” for coffee). “It was so natural for me to do it. You’d be amazed how often I get asked the same questions. How do you work? How do you interview? I’m going to write a longer memoir, but I thought I’d just give a few glimpses. I said: I’ll just sit down and write what I can remember. The whole thing only took nine months. It poured out.” He sounds faintly amazed. Ordinarily, his books run to about 800,000 words; this one is a tenth of that. “We edited it in two days!” He closes his eyes. “And now I’m back in Vietnam [with LBJ].”

Working contains plenty of advice for biographers, including the value of silence – “SU” he writes in his notebook before interviews, which stands for “shut up” – though a writer should, he believes, also be willing to go back to his witness again and again, squeezing him like an orange until the pips shoot out; Caro talked to LBJ’s aide Horace Busby, who died in 2000, no fewer than 22 times. (“I ask people over and over what they saw, and what they heard. They get really fed up. But often people don’t know what they know.”) No, it may not be possible to look at every document relating to your subject – the LBJ library contains 45m pieces of paper – but Caro has never forgotten the words of his editor at Newsday, delivered when he was just a rookie: “Turn every goddamned page.” There is no substitute for going through the files, and not only because they will almost certainly yield secrets; to hold a contemporaneous document in your hand is to be brought magically closer to what he calls “genuineness”, a form of time travel so compelling that even now, when he’s sitting next to a library cart piled high with his requests, he sometimes finds it difficult to keep track of the hour.

The best bits of Working, however, are its set pieces, each one more indelible than the last. The time he slept out in the Texas Hill Country, the better to understand the extreme loneliness of LBJ’s mother. The time he took LBJ’s brother Sam Houston back to the family ranch in Johnson City and finally heard of the terrible rows the future president used to have with his father. The time he interviewed LBJ’s wife, Lady Bird, unable to meet her eye as she talked to him of her husband’s love affair with a beautiful young woman called Alice Glass. Most gripping of all, there is an account of his visit in 2000 to the home of a mortally ill Georgia senator, Herman Talmadge, one of the good old racist boys who raised LBJ to the Senate, believing him to be on the side of the south, only to watch, horrified, as his civil rights legislation passed into law. “It was a brilliant moment,” Caro says, unable to resist taking up the story. “This immense power. You drive along Herman Talmadge Highway, and you exit on Herman Talmadge Boulevard, and you arrive at this house with the tall white columns, and a black man in a waistcoat comes to open the door…” He asked the elderly senator what he had believed Johnson thought about the relationship between whites and blacks in segregated America. “Master and servant,” came the reply. Then he asked him how he felt about the Civil Rights Act of 1964. “Disappointed. Angry. Sick,” said the senator, after his own long silence.

“LBJ made these guys believe for 20 years that he believed something he didn’t believe at all,” says Caro, delight creeping over his face. “When people say that power corrupts… I don’t happen to believe that. Power reveals. When you’re on your way up, you have to conceal what you intend to do. Once you get power, then you see it, what he really wanted to do.” Much of the fascination of LBJ lies – somewhat awkwardly in an era when human beings must either be good or bad, and nothing in between – in the fact that the same man who does this “miraculous” thing for black Americans is also the president that escalates the war in Vietnam; that he is – together, we agree on two adjectives – both wondrous and appalling. “People are always asking me: do you like Lyndon Johnson?” he says. “But those terms aren’t even relevant. I’m in awe of Lyndon Johnson. He takes Kennedy’s bills, which are going nowhere, and he passes them. How does he do it? If Johnson hadn’t been president, we wouldn’t have had Medicare to this day. But, on the other side, there is Vietnam, and very little is redeeming about that.” Of course the good and bad in him are related: “As I will show, there’s a straight line between the two.” How does he plan to do that? He grins. “Well, I haven’t quite written it yet, so I’m going to take a pass on that question for now.”

For Caro, the key to understanding Johnson lies in his childhood. “That terrible childhood. The loneliness and the poverty. His father transformed into the laughing stock of the town [having hubristically bought it back, Johnson’s father subsequently lost the family ranch, a property that anyone with eyes could see was on infertile land]. Every month they were afraid the bank would take their house. He came out of that with his character formed; the fire was too hot.” What, then, is Caro’s key? What makes him – the obsessive researcher, the completist writer – the way that he is? Will I find it in his childhood? What’s funny is how little he seems to know about himself (I do not believe he is only being coy or wilfully obtuse). “It’s an interesting question,” he says, almost wonderingly. “But I don’t know the answer. I’ve always been the same way.”

Caro was born in New York City in 1935; his mother was a native New Yorker, his businessman father an immigrant from Poland who spoke Yiddish with his friends. “He taught himself to read and write English by copying out whole stories from the New York Times by hand,” says Caro. His mother died when he was only 12, but his father was still alive when, in 1974, he published his first book, The Power Broker (his only book, besides Working and the four volumes of LBJ), a biography of Robert Moses, the so-called “master builder” of New York. It would go on to win the first of his two Pulitzer prizes (the second was for Master of the Senate, the third volume of The Years of Lyndon Johnson). Was his father proud? “Yes. He was a very silent man. But he was proud.”

He always wanted to be a writer. To this day, he still has regular dinners with the guys who worked with him on the Horace Mann Record (Horace Mann is the private school he attended in Riverdale, the Bronx). At Princeton, he neglected his studies, preferring to devote his time to its daily newspaper. He began his career as a reporter at the New Brunswick Daily Home News, eventually graduating to Newsday on Long Island. “I loved being a journalist,” he says. “I miss it.” It isn’t lost on him that while he once had a reputation for speed, he’s now notorious for his refusal ever to be hurried: “It’s sort of opposites. In newspapers, you’re always on a deadline. I was really fast. I was maybe the fastest rewrite man. Newsday had all these editions: East End, Riverhead, Mid Suffolk, East Nassau, Central Nassau, Queens. You would have to write six leads for the same story. I could do that. I liked doing that. But you never had a chance to think things through.”

With books – he left Newsday to write The Power Broker – the research is the thing that really takes the time. But even the writing he does with a circumspection so unstinting, you wonder it doesn’t drive him mad: “I write in longhand in these legal pads. I do a lot of drafts like that. Then I go to my typewriter, and I do a lot of drafts like that, too.” (The endpapers of Working are formed from pages of these typewritten drafts, each one marked with thick, black crossings-out and scribbled additions so numerous, they’re close to indecipherable.) Finally, there is the editing process. Robert Gottlieb, at Knopf, who has done the job since 1971, cut 350,000 words from The Power Broker, a knife attack from which Caro is still recovering: “When people ask me what’s missing, it really hurts.” That’s why he decided to write LBJ as several volumes (three, originally). That way, nothing major would have to go – and perhaps nothing minor, either.

Working is dedicated to his “beloved” Ina, his researcher on all the books. She is stalwart: as single-minded as him, in her way. When they were broke during the writing of The Power Broker – his advance had run out – it was Ina who sold their Long Island home without even telling him. When they moved to live in the Texas Hill Country in 1978, to get to know the crucible in which Johnson was formed, it was Ina who befriended the taciturn local women, turning up at their doors with the homemade fig jam she’d taught herself to make especially for this purpose. Is she his right arm? “That’s a hard question.” For some seconds, he is silent. “I feel I would… I feel… I can’t imagine not doing something that I think is important. But at the same time, she is the only person I’m able to trust. I call her the whole team.”

He looks almost sheepish. “You know, when I’m writing, I really concentrate. I know I’m hard to be with like that. You get all wound up. I used to have a drink when I got home, but I can’t do that now [following an illness, he was advised to give up alcohol]. Now she says: ‘Boy, I wish you could drink again.’ She’ll tell me: Don’t speak to anyone for an hour after you finish work.’ I know that’s good advice.” They go out a lot: the movies, musicals, dinner with old friends. But it’s not easy re-entering 21st-century New York after a day spent in, say, 1960s DC. It’s like being a diver. Surface too quickly, and you get the bends.

He doesn’t worry about the future of biography, though it seems clear that he is the last of the last in terms of producing multiple volume lives. His main concern is that people do not forget that the quality of the prose in the writing of history and biography is as important as it is in fiction. “I have no trouble in understanding why [Edward] Gibbon endures,” he says. “Look at the writing! He is great.”

Caro’s own prose makes me think of waves: in the paragraphs roll, grandiose as anything, crashing against the shore as he winds them up with a last, very short sentence. “Well, that’s from Paradise, um…” He shakes his head. “I don’t compare myself with Milton, but great works can be models. He [Milton] has these long lines about Satan falling and falling and then, suddenly, the rhythm changes. I try to do things with rhythm. In the second volume, Johnson is campaigning in Texas in a helicopter, and he’s so desperate. I wrote on an index card: is there desperation on this page? I meant in the rhythm. I want to reinforce the reader’s understanding with that rhythm.” What about facts? Does he fear for those in the age of Trump? The truth no longer seems to matter to some. “Of course it’s dangerous. People who believe there aren’t facts… it’s irrational. There are facts, and the more of them you collect, the closer you come to whatever the truth is.”

And so, finally, we come to the elephant in the room. If writing a book is like climbing a mountain, has he crested the peak of the final volume of LBJ? Is he on his way back down the other side? This is delicate, I know, but does he feel the clock ticking? His eyes close once again. “I have a long way to go,” he says. “I can hear the clock ticking, but the important thing to me is to ignore it. If I was to rush, what would be the sense?” The responsibility of his project – a project to which he has devoted almost half his life – never leaves him. “Almost all the people involved are dead now. They’re leaving you to tell the story, so you’d better get it right.”

But it’s more than this. These books do not only relate the life of one man. They comprise a history of America itself from the beginning of the 20th century until the middle of the 1970s. “The second volume is about the election LBJ stole [in 1948, when he won election as a senator by just 87 votes; Caro found proof that ballots had been fraudulently certified]. People often ask why I spent so much time on a stolen election. Well, the answer is that they poison democracy at its root, and they’re not rare. They’re quite common in American history. Then there’s what I write about the Senate: what it was, and what it can be. No president has made the Senate work since Johnson. I want to show what political power can do – the horrible things, but also the beneficent things. People have forgotten the beneficent effect political power can have on their lives. I want readers to feel that.”

I hand him my copy of Working, and he signs it, quickly and without hesitation or crossings out. Talking of these responsibilities has brought an end to the reminiscences. “I have to get on,” he says, his hand fluttering. It’s an odd and paradoxical thing to write, but the atmosphere in the room is suddenly urgent, half a precious day and all the words it represents having already long since run through the hour glass.

• Working by Robert Caro is published by Bodley Head (£20). To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99