obert A. Caro is easing himself back into harness. A restorative summer in Paris over, a warm-up spell in East Hampton done with, he's set to begin the fourth and, he swears, final volume of his biography of Lyndon Baines Johnson.

There's no rush: he has already spent 25 years on the Johnson book. The third volume alone, published in April, took 12 years. Titled "Master of the Senate," it topped the New York Times best-seller list in May and remained there for 11 weeks. Even so, he is eager to get back to work. He has already produced some 2,500 pages ó about 1.5 million words ó on Johnson, but figures he has another 1,000 pages to go.

Every day he heads off to his office, a drab room in a nondescript building on West 57th Street, a 10-minute walk from his home on Central Park West. No name on the door, no secretary. One wall is lined with books, another with file cabinets. Yet another is covered ó its full 22-foot length ó with the chapter-by-chapter outline of Vol. 3. "That's where I'll put what I'm doing now, he said, "the outline for Vol. 4."

Though he spends his days at the office alone, he goes to work in suit and tie. "To remind me that I'm working," he said.

At the moment his business attire includes a sling. Two weeks ago he tore the rotator cuff in his left shoulder swimming in the surf off Long Island. Pencil-and-pad interviews are out for now, but he can scratch away at his outline, and a new voice-actuated computer should help transcribe documents.

One attraction of a summer in France was that Johnson never spent much time there. The Caros spent June and July eating well, watching old flicks at a local grind house and recharging a long affair with the City of Light. They did do some work. Or Ina Caro did. Her husband's entire research staff, in-house critic and social director for most of the year, in France she is also a working author in her own right. Her first book, "The Road From the Past" (Doubleday, 1994), was a guide to historical France spiced with an occasional tip on a good inn or restaurant. This summer she worked on a guide for brief forays out of Paris. He wrote nothing in France.

Mr. Caro's name and Johnson's have been inextricably entwined for more than a quarter of a century. And yet there was a time, in the early 1970's, when the book might not have been done. In 1970, when Mr. Caro was still working on "The Power Broker," his biography of Robert Moses, he switched publishers from Simon & Schuster to Alfred A. Knopf. In exchange for a more generous advance, Knopf asked for two books, the one on Moses and a biography of Fiorello La Guardia, New York's colorful mayor from 1934 to 1945.

Though he agreed at first, he realized he would be going over a lot of the same ground he had covered in "The Power Broker." "I wanted to write about political power and how it was acquired and used," Mr. Caro said. "With Moses, I'd done that on the city and state level. I decided I wanted to do Lyndon Johnson. And I wanted to do him in two volumes."

One day, a call came from his Knopf editor, Robert Gottlieb. "He asked me to have a sandwich with him at his office," Mr. Caro said. "When I got there he said: `Look, you don't want to do La Guardia. Why don't you do Lyndon Johnson? And do it in two volumes.' "

Robert Allen Caro was born in Manhattan on Oct. 30, 1935. He attended P.S. 93 in Manhattan, the Horace Mann School in Riverdale and graduated from Princeton in 1957 with a degree in English literature. His mother died after a long illness when he was 11. His relationship with his father, Benjamin Caro, was strained. "As a kid, I spent a lot of time reading on a bench in Central Park," he said. "And I always wrote." In the sixth grade he put out a class newspaper on a mimeograph machine. At Horace Mann he gravitated to the literary magazine, but in his senior year edited a weekly paper there. "I wasted Princeton," he said, "spending my time on sports and a column in the school newspaper. It was called "Ivy Inklings," he said, wincing. "I still have a bound volume of it, and it's embarrassing." Still, he agreed to edit The Daily Princetonian only when he was told he could keep the column.

But "wasted?" Not entirely. The eminent R. P. Blackmur was one of his teachers. "I'd wait until the last minute to do my papers," Mr. Caro said, "then bang them out at top speed." Blackmur was not impressed. "The trouble with you, Mr. Caro," he said, "is that you write with your fingers." No longer.

Mr. Caro writes in longhand. Then he rewrites in longhand. Then he rewrites again on an old electric typewriter. This version goes to a typist and then to Bob Gottlieb at Knopf where the real editing begins. "He will fight as hard for a semicolon as for a section of the book," Mr. Gottlieb said. Professor Blackmur had made his point.

The Caros met at a Princeton social for Jewish students and were married the day after he graduated. They have a son, Chase, and three grandchildren. His first job out of Princeton was as a reporter for the The New Brunswick Home News in New Jersey, where one of his more unsavory tasks was writing speeches for the County Democratic Organization. One day he walked out. Next came Newsday on Long Island, where an old-school editor, Alan Hathaway, took him on despite misgivings about buttoned-down Ivy Leaguers.

A meticulously reported story on the development of Mitchell Field airport mollified Mr. Hathaway and led to a succession of critically acclaimed projects. One, "Suffolk: The Sick Giant," a six-part series in 1963, led to the creation of a planning board for Suffolk County and a Nieman Fellowship at Harvard University. At Harvard he soon concluded that the only way to explain how politicians influenced urban development would be to write a biography of the developers' developer, Robert Moses.

Mr. Caro expected to be back at Newsday in 12 months. It took seven years. Simon & Schuster's advance was so meager he considered keeping his day job. Then a Carnegie Fellowship made it possible to write full time. Innocently, he told his wife they would be celebrating the finished book in Paris in nine months.

In what has become a Caro tradition, the book grew and grew. Money was tight. Selling their house in Long Island netted enough for a year. They took an apartment in Riverdale, and Mrs. Caro taught school in Yonkers. Five years into "The Power Broker" Mr. Caro asked Simon & Schuster for the rest of his advance and was turned down. "I remember walking up Broadway," he said, "wondering what I'd tell Ina."

But his contract had a five-year "out clause." If his editor left, so could he. When the editor did, he did, too. For the first time, he hired an agent, Lynn Nesbitt. "She read half the book," he said, "and simply asked me what I wanted for it. I was stunned." She told him: "I can get it for you with one phone call. You can stop worrying. Everyone in New York knows about this book."

Mr. Caro said: "She made appointments with four editors. The fourth was Bob Gottlieb. `This is a great book,' he told me, `but it needs a lot of work.' " Ms. Nesbitt showed the manuscript to William Shawn, the editor of The New Yorker, who agreed to run 140,000 words in the magazine. "That check got us to Paris for the first time," Mr. Caro said.

"The Power Broker" was published in 1974. "The Path to Power," the first volume of "The Years of Lyndon Johnson," was published in 1982 and excerpted in The Atlantic Monthly. The second volume, "Means of Ascent," appeared in 1990 and, like the most recent volume, was excerpted in The New Yorker. When Mr. Gottlieb succeeded Shawn at The New Yorker, he continued to edit Mr. Caro.

"I've been truly lucky," Mr. Caro said. "For 30 years I've had the same editor, the same agent, the same assistant editor, Kathy Hourigan, and the same research team, my wife." "Master of the Senate" ends with Johnson's winning the vice presidency in 1960. Still to come are the years of his presidency, the Vietnam War and, ultimately, his death in January 1973.

Gone are Mr. Caro's concerns about money and acceptance. "The Power Broker," now in its 34th printing, is required reading in more than 200 schools and colleges. At this point his competition is time or, more pointedly, mortality.

In a C-Span interview earlier this year, he spoke of the satisfaction that comes from finishing another section of the book, implying that the parts can stand as a complete work even if he can no longer add to them. The fourth and final volume should take five years, Mr. Caro said, acknowledging ruefully that his past estimates have at times been considerably wide of the mark.

During his Neiman days at Harvard Mr. Caro injured his back playing basketball and was bedridden for almost a year. "Ina was my eyes and ears," he said. She also found help. Few remember that "The Power Broker" was dedicated not just to his wife but also to Janet Travell, the physician who treated President John F. Kennedy's back problems. Dr. Travell worked extensively with Mr. Caro, even designing a special desk for him much as she had prescribed Kennedy's rocking chair. With her help, Mr. Caro's condition improved dramatically, but long hours at his desk can still be painful.

"I've done much of the interview work already," Mr. Caro said of the final volume. Just as he lived in West Texas to help with Vol. 1 and took an apartment in Washington to experience the Senate first hand, he plans to live in the Deep South to experience the impact of Johnson's civil rights legislation, and in Vietnam for a time to learn what it was like to live through what became Lyndon Johnson's war.

And after Vol. 4? Mr. Caro only smiles. "I'm superstitious," he says. "I don't want to jinx myself." A clue: Jackie Robinson was one of his true heroes. Nothing more said.

There are literary heroes, too, among them Sir Edward Gibbon, whose "Decline and Fall of the Roman Empire," Mr. Caro notes, comes to six volumes. Beyond prolixity, the two share another trait: longhand. The Duke of Gloucester's oft-repeated jibe at Sir Edward (upon receiving the second volume from the historian), fits both.

Bob Caro is again at his desk in New York, alone with paper, pen and his own First Consul, Lyndon Johnson. "Scribble, scribble, scribble, eh Mr. Caro?"