The office on Manhattan's West 57th Street is empty. The brass plaque on the door says "Robert A. Caro, Inc." Within minutes a puffing and harassed Caro comes running off the elevator, profusely apologizing for being three minutes late. A friend had taken ill the night before, and he has still to get in touch with him. Inside the office, a big block of a room, Caro finally contacts his friend, who is feeling better. He then insists that PW's correspondent surrender his jacket, which he hangs up himself, and orders coffee from the local deli. He sits behind two huge desks fashioned into an "L." On one desk is a Smith-Corona electric typewriter, circa 1969, and on the other, a foot-high pile of single-page proofs of Master of the Senate, his third volume in The Years of Lyndon Johnson series.

Master of the Senate--like his two previous tomes about America's 36th president, Lyndon Baines Johnson, The Path to Power (1982) and Means of Ascent (1990), and his Pulitzer Prize-winning dissection of Robert Moses, The Power Broker (1974)--is about one thing: power. "I've never been interested in writing biographies," says Caro candidly as he passes out the newly delivered coffee to his guest. "My interest is in political power and how it shapes our lives. We learn in college that political power in a democracy comes from being elected at the ballot box. Where does Robert Moses get his power? He's never been elected to anything. It came to me as a realization that I was really just full of bullshit because I'm trying to explain what political power is, in traditional terms, and here's a guy who's never elected to anything, and he had more power than any mayor or governor combined. And he had it for almost half a century."

The Power Broker rattled a lot of skeletons and opened a lot of eyes. But the story behind The Power Broker is a story of heroism--the heroism of its writer. "All the time we"--the "we" signifying Caro and his wife and research collaborator, Ina--"were doing it, we were broke," confesses Caro. "I never thought I'd get to do another book. I was just trying to get back to Newsday," where he had been an investigative reporter. "I just wanted a paycheck." The Power Broker was originally contracted by Simon & Schuster in 1967. When asked how much his advance was, Caro painfully says, "I can't remember. It was either $5,000, of which they gave me $2,500. Or $10,000, of which they gave me $5,000. Whatever it was, it wasn't enough to live." Things got so bad that Caro's wife was forced to sell the family house on Long Island. "Ina loved that house," says Caro, the memory of the pain and the love of his wife clearly visible on his face, "and without even telling me, she put it on the market and sold it.

"I said to Simon & Schuster, 'Can I have the other half of my advance?' And they said, 'No, you don't understand. Nobody is going to read a book about Robert Moses.' " Soon after, Caro's editor left and an "out clause" in his contract made him a free agent. "At that time," he recalls, "I only knew one writer in the whole world. I knew no agents. I'd never met an agent. I knew no editors basically, except the guy at Simon & Schuster who wouldn't return my calls for long periods of time. But I knew I needed an agent. This one writer gave me a list of agents and Lynn Nesbit was on that list." As they say at the end of Casablanca, this was the beginning of a beautiful friendship.

"From the first minute, Lynn understood me. 'Well, I read this book, it's a great book. What do you want?' That's the way Lynn is, she's very direct. I told her I wanted enough money to finish this book. And she said, which I remember to this day, 'Is that what you're so worried about?' I said yes. 'Well, you can stop worrying.' I thought nobody cared about the book. 'Everybody in New York knows about this book,' she said. 'I can get that for you with a telephone call. You can stop thinking about that. It seems to me the only thing you care about is writing, so I have to find you an editor that you can work with the rest of your life.' "

Nesbit arranged for Caro to meet four editors, one of whom was Bob Gottlieb of Knopf. "Three of them took me to lunch and told me that they could make me a star," says Caro with a laugh. "Gottlieb didn't take me to lunch--he never had lunch--he said, 'This is a great book, but it needs work. This is what has to be done.' So, of course, I went with him and I've been with him ever since."

Caro tells some delightful stories about his pursuit of Robert Moses and Moses's efforts to block his path. He especially had trouble getting access to Moses's papers, which Moses kept under lock and key. Things looked pretty bleak until Mary Perot Nichols--then PR person for the Parks Department and later a columnist at the Village Voice--rode to the rescue. "So Mary said to me," recalls Caro, 'I know you're doing a book on Robert Moses, how are you doing with his papers?' I said I'm not seeing any of his papers." It turned out that Moses dictated to his empire in duplicate--and Perot knew where the carbon copies were buried.

"They were at the 79th Street Boat Basin," recalls Caro with good humor. "There is this huge subterranean space. Along the far wall was this long, long row of four-drawer filing cabinets that had not been opened in years. When Moses retired as parks commissioner, they simply shipped his papers down there from wherever they were, and here were all the carbons. There was only one light bulb. Mary Nichols gave me a key and Ina would come with me because we could work faster. And the 'parkees,' you know, the little guys in green, they realized I was doing something that the commissioner, as they called him, wouldn't like. So what they would do is that every time I had to go to the bathroom, they would take the light bulb out," he says laughing. "So I started arriving in the morning with my typewriter and an attaché case with four 150 watt light bulbs!"

PW asks Caro what the difference was in the way Moses and LBJ wielded power. "Totally opposite," says Caro with passion. "Lyndon Johnson will put his arm around you. It's interaction with human beings. In this book, I call him a 'reader of men.' The greatest reader of one-on-one. With Johnson, it was manipulation and domination and inspiration of human beings. Moses is the opposite. He didn't want anyone touching him. He didn't want to be touched. That's sort of symbolic."

Caro's less-than-admiring approach to Johnson in the first two volumes turned off several LBJ confidants--including Lady Bird Johnson and former aide Jack Valenti. However, it had the opposite effect on John Connally, who was the closest thing LBJ had to a political alter ego. Caro went hard after Connally without any success. One day in the mid-1980s, the phone rang and it was a mutual friend in Washington with a piece of good news for Caro. His friend had been at a dinner party with "the Governor"--"they all called Connally the Governor"--and he's "reading your book."

"I said something like 'I doubt that,' " Caro recalls. "Some time thereafter the phone rings and it's Connally, and he says, 'You know, Mr. Caro, I'm really sorry that you wrote your first volume without trying to talk to me.' So I had enough presence of mind not to get angry." Caro has to stop and laugh at the memory. "He said something very complimentary-- 'You got him right on the head. And I don't want you to do the second volume without talking to me.' " Caro spent much time with Connally at his Texas ranch gathering invaluable material on LBJ.

Much of Master of the Senate focuses on Johnson's deft passage of the 1957 civil rights bill through the Senate. "In the field of civil rights," says Caro unequivocally, "he did more than any president--not since Roosevelt, since Lincoln. He's the greatest champion that black people have had in this century. I am sure we wouldn't have those Civil Rights Acts today if it wasn't for him. Right after him the country turned to the right."

Even today, many question LBJ's intentions on the 1957 bill, arguing that Johnson had to pass a civil rights bill if he ever wanted to be taken seriously as a presidential candidate. Caro, who focused on the petty Johnson and the election-stealing Johnson in previous volumes, isn't buying any of that. "He was possessed of a compassion that was true and real and deep," says Caro with quiet sincerity. "I believe that. All his life he wanted to help poor people and particularly poor people of color."

This questioning of motives forces Caro back to his favorite subject--power. He quotes Lord Acton's theorem: "All power tends to corrupt; absolute power corrupts absolutely." "The more I work the less I'm convinced that that is really true," says Caro thoughtfully. "But I will tell what I believe is really true. What power always does is reveal. Power reveals. Now, Johnson has power and what's the first thing he does? He sets out to pass the Civil Rights Act. Did he do it for a mixture of motives? Absolutely. But he really wanted to do it."

Recently, there has been a lot of controversy about the scholarship and plagiarism charges that have been lobbed at historians Stephen E. Ambrose and Doris Kearns Goodwin. Martin Arnold of the New York Times recently weighed in and concluded, "No one would waste a moment checking Robert A. Caro, who invests years in researching and writing each of his books." Asked about the brouhaha, Caro turns quiet and speaks deliberately. "The one thing I learned in my work is that you don't really know anything unless you find it out for yourself. Don't trust anything." Caro goes on to give his philosophy on research. . "I will say this--I could never use a team of researchers," he freely admits. "Years ago, when I was doing The Power Broker, I got this grant that included an assistant, so I hired one, but after a couple of weeks, I said, 'I'll be glad to pay you for a year--just don't come in!' I have to look at everything myself. Ina is great, and Ina knows how I think, but even when she finds something, I have to look at it myself."

PW spies that old Smith-Corona and is forced to ask Caro if he's a Luddite, eliciting a laugh and a shrug. He admits to writing his first three or four drafts in longhand, then going to his trusty typewriter and typing triple-spaced on legal-sized paper for editing purposes. "I write in longhand for a reason," he says. "I write in longhand to slow myself down."

Caro, who lives in the West 60s, walks south through Central Park every day to his office. "I have two lives," he says, "because part of my life is writing. In your research life your hours don't depend on you. Whenever someone wants to give you the interview, that's when you're going to start. But when I'm writing, I try to get her by 8 a.m. I'm usually here earlier. The fact of my life is, with each chapter, as you get more wound up near the end, you get more and more excited. You're getting up earlier and earlier. Sometimes," he says with a rueful laugh, "you're getting here really early in the morning."

"Sometimes you really get down as to whether books will endure," says Caro thoughtfully. He shifts in his chair and points out the window in the direction of Columbus Circle. "As you look out the window here, you see this big building, well, that's on the site of Moses's New York Coliseum. Moses really thought that it was going to make him immortal. He was always saying, the life of the suspension bridge is infinite. And he used to say about The Power Broker --this is his line--'how long will it last? It will disappear before you know it.'

"So," says Robert A. Caro with a huge smile of satisfaction that borders on glee, "when they started tearing down his Coliseum, I said, 'Listen, books do last.' "