NEW YORK (NYT NEWS SERVICE ).- Robert Caro is famous for colossal biographies of colossal figures. “The Power Broker,” his Pulitzer Prize-winning life of Robert Moses, weighed in at nearly 1,300 pages. His as-yet-unfinished biography of Lyndon B. Johnson — he likes to call the volume-in-progress “the fifth of a projected three” — totals 3,444 pages and counting.

The books are already monumental. And now Caro is getting monumental treatment himself.

The New-York Historical Society has acquired Caro’s papers — some 200 linear feet of material that will be open to researchers in its library. And just as important to the 84-year-old Caro, it will create a permanent installation in its museum galleries dedicated to showing how he got the job done.

“It’s like a true weight has been lifted from my shoulders,” he said last week in his office off Central Park West, where he was surrounded by hulking filing cabinets, piles of heavily scribbled-on legal pads and — tantalizingly — a wooden box holding typed pages of the eagerly awaited final Johnson volume.

In discussing the plans for the permanent exhibition, he repeatedly stressed the permanent part.

“With most archives, there’s a big splash, then two or three months later, it’s time for the next,” Caro said. “But I wanted something that wouldn’t go away.

“I want people to be able to see how I gather my material and how I turn it into books, how I write,” he continued. “In my opinion, the quality of the prose is just as important in nonfiction as in fiction.”

The archive will be among the largest of a single individual in the historical society’s collection. It includes research notes, drafts, annotated news clippings, correspondence and other documents, from once-classified memos excavated at the LBJ Presidential Library to at least one artifact literally coaxed out of a secret trunk.

The archive also holds Caro’s transcripts from thousands of interviews with subjects ranging from Moses and Lady Bird Johnson to obscure figures he tracked down — and sometimes interviewed over and over — in a dogged effort to nail down seemingly every discoverable detail.

Louise Mirrer, president and chief executive of the historical society, whose other acquisitions of 20th-century material include the papers of financier Felix Rohatyn and the vast archive of Time Inc., called the richness and scope of Caro’s papers “astonishing.”

“This is an archive that will illuminate the 20th century through two outsize figures, Lyndon Johnson and Robert Moses,” she said. “It’s also the record of an extraordinary writer and thinker. Bob Caro is a historian whose methodology is of equal importance to the actual materials in his archive.”

Caro has been talking a lot about those methods lately. Last spring, he published “Working,” an anecdote-rich collection of essays and interviews offered, he said, as a kind of promissory note on a proper memoir, which he has already started outlining.

And the final volume of the Johnson biography? Don’t worry, he’s working on it. In fact, he said, checking the sheet of paper in the Smith-Corona Electra 210 on his desk, he has typed 604 manuscript pages so far.

Asked where he was in the story, Caro paused, looking mildly stricken. But he allowed that he’s currently on a section relating to the creation of Medicare in 1965, with the debacle of Vietnam and Johnson’s decision not to seek reelection in 1968, to come. (Johnson died in 1973.)

“It’s going to be a very long book,” he said.

Caro — who will keep some of his materials in his office until he’s done with the biography — is protective of details of work in progress. But looking into his notebooks and file drawers (sample tab: “Johnson Personal: Ruthlessness”), he was expansive, offering anecdotes within anecdotes about the events and people he was trying to reconstruct and about how he and his wife and longtime research partner, Ina Caro, found sources, secured interviews and tracked down documents. (Neither the historical society nor Robert Caro would disclose financial details of the acquisition, though Caro said the price was “more generous than I expected.”)

Michael T. Ryan, director of the historical society’s library, recalled being surprised by the unusual number of primary-source documents tucked away in various files, boxes and even briefcases.

“We have here — possibly — an extraordinary cache of material for which there is no other copy,” he said.

And then there is everything that was cut from Caro’s already-gargantuan books, like some 300,000 words axed from “The Power Broker” by his longtime editor, Robert Gottlieb, including extensive material on figures like Jane Jacobs and Al Smith.

“All I can really think about as I go through the files is what I left out,” Caro said. “These archives,” he added, “contain information on worlds that have completely vanished.”

Caro’s current office, which he moved to last summer, is itself a kind of museum of a vanishing analog world, down to a closet holding some of his 11 stockpiled Smith Coronas, ready to be cannibalized for parts.

An elaborate typed outline of his book in progress is tacked to corkboards lining the walls. Caro — who writes his first drafts by hand on legal pads — pulled out a thick binder to show how he writes each sentence from the outline on a loose sheet of lined paper. Next, he lists interviews and documents that support the point, all indexed according to a complicated, idiosyncratic system.

But the real guts of the operation are in an adjacent room, in the files and cabinets holding his interview notes and other documents, including diaries and notes given to him over the years by Johnson associates and other journalists.

Caro popped open a cabinet filled with reporter’s notebooks and pulled a purple Scribbletex pad labeled “LBJ I”; inside were his first interviews, with Lady Bird Johnson and Sam Houston Johnson, the president’s brother.

He then pulled out “LBJ XXXII,” flipping through pages covered with neatly handwritten notes and, on one page, a list of cryptic numbers.

“I hate flying, so I always write out how many minutes are left,” he said, laughing.

One of his rules — Caro does not use a tape recorder — is to always type up his interview notes before going to bed, so as not to forget facial expressions or other details. “He can’t seem to sit still,” reads a note atop a transcript of one of his interviews with Moses.

On a folder holding his 22 formal interviews with Horace Busby, a longtime Johnson aide, Caro pointed out a long-ago scribbled reminder: “With Busby the key thing is to SHUT UP! He doesn’t want to hear one word from you. His eyes get bored the minute you open your mouth to say anything.”

As the interview drew to a close, Caro pulled out the photocopy of one of his most famous discoveries: an unpublished memoir by Luis Salas, a Texas election judge, explaining how he had helped Johnson eke out an 87-vote victory in his 1948 Senate race, by assigning him votes cast for his opponent.

In “Working,” Caro describes how in 1986, a decade into work on the Johnson biography, he finally tracked Salas down in a mobile home near Houston. Caro was seeking hard proof that the election had been stolen. In the middle of their interview, Salas suddenly fished the manuscript out of a trunk, saying, “I have written it all down.”

In his office, Caro read aloud a passage that might serve as a gloss on his own exhaustive, still-unfinished work.

“Maybe I pass away before I see my book published,” Salas wrote, “but someday it will come out because it is part of the history of the United States, and people have the right to know the exact truth.”