Those sessions, nearly 80 in all, are the fodder for a new book by Mr. Branch, tentatively titled “Wrestling History: The Bill Clinton Tapes,” that Simon & Schuster plans to publish in late 2008, the publisher said today.

Mr. Branch, the Pulitzer Prize-winning author of “America in the King Years, 1954-1968,” a trilogy on the life and times of the Rev. Dr. Martin Luther King Jr., said he intended to write about what he called an extraordinary and unprecedented series of sessions that began as an oral history project when Mr. Clinton was still the president-elect.

They are also the product of a friendship between Mr. Clinton and Mr. Branch that dates back nearly 40 years, to when the men met at antiwar meetings in the thick of the Vietnam War and collaborated on Senator George S. McGovern’s 1972 presidential campaign in Texas.

Mr. Branch, 60, is currently winnowing 2,600 pages of raw material into a book that he plans to begin writing within a few weeks, he said.

“I’m not calling this a biography of Clinton or a history of the administration,” Mr. Branch said in a telephone interview from Baltimore, where he lives. “It is what it was like to live through it that way, sitting alone with him, talking about the presidency as he saw it, right in the moment.”

David Rosenthal, the publisher of Simon & Schuster, said the material “is wide-ranging, largely unguarded and gives tremendous insight into the thought processes and real-time concerns of a sitting president.”

The recording sessions began in 1992 with a call to Mr. Branch from Mr. Clinton’s office, asking if he would come meet with the new president-elect. Mr. Clinton suggested that his incoming appointees would not keep vivid enough records of his administration and agreed to regular recorded “diary sessions,” Mr. Branch said.

A few years into the sessions, Mr. Clinton suggested that Mr. Branch could take advantage of the reams of material at his disposal. “While we were in the middle of it, he said, ‘You know, one day you might want to write a memoir about this,’ ” Mr. Branch said. “ ‘I don’t think anybody’s ever done anything like this with a sitting president.’ ”

Mr. Branch said he bore witness to Mr. Clinton’s ever-changing moods throughout his tumultuous presidency. The night before Mr. Clinton sent American troops into Haiti in 1994, Mr. Branch said, “He was writhing, almost.”

Then, after his landslide re-election in 1996, Mr. Clinton was in a very good mood, gleefully reciting numbers from precincts all over the country.

The men bonded over their roots as Southerners who witnessed the civil rights movement and later joined the antiwar movement, and their friendship deepened throughout the presidency. Mr. Clinton occasionally asked Mr. Branch for advice, and Mr. Branch responded with detailed memos. He was an occasional speechwriter to Mr. Clinton, helping craft both of his inaugural addresses. (Mr. Branch’s wife, Christina Macy, was a speechwriter for Hillary Rodham Clinton from 1998 to 1999.) In 1996 he wrote an article in Esquire titled “Clinton Without Apologies” that zealously defended Mr. Clinton.

Mr. Rosenthal said while the book would be “intimately reflective” of Mr. Clinton’s thinking, the former president is not a participant in Mr. Branch’s book. “I think he will accurately depict what was said,” Mr. Rosenthal said. “But they’re old friends and I think they have an affectionate relationship, to say the least. So I think this is not so much in any way to discover any deep, nasty skeletons in Bill Clinton’s closet. Nor do I think it’s meant to polish the apple.”

Mr. Branch’s bona fides as a historian are well established. The first volume of his King trilogy, “Parting the Waters,” which was published in 1988, won the Pulitzer Prize for history. The third volume, “At Canaan’s Edge,” was a finalist for a National Book Award last year.

But he may share his subject’s difficulty in writerly self-restraint. Reviewing “At Canaan’s Edge” in The New York Times in January 2006, Michiko Kakutani wrote, “Mr. Branch has ranged far and wide across the political and social landscape, often resorting to newsreel-like summaries of developments, while pelting the reader with incidents and facts in the place of analysis and perspective.” (Mr. Clinton’s 2004 memoir, “My Life,” clocked in at 957 pages.)

The tapes produced by the recording sessions could be used for another memoir by Mr. Clinton. He kept all of the tapes, squirreling them away in his sock drawer after each session. Mr. Branch will rely on his own notes and recollections of the conversations, which he routinely recorded during his hourlong drive back to Baltimore from Washington.

Mr. Clinton labored to keep most of the White House staff unaware of the recording sessions by holding them late at night in his second-floor office, known as the Treaty Room, and once at Camp David during Middle East peace talks.

Mr. Branch, for his part, also tried to keep the project secret. In the past he has deflected suggestions that he would collaborate with Mr. Clinton on a memoir, even insisting that he had ruled out the possibility. (He said he didn’t know yet if he would allow Mr. Clinton to read the manuscript before publication.)

In an interview with The New York Times in 1998, Mr. Branch acknowledged that he and Mr. Clinton had discussed the idea of writing a presidential memoir. “After a while, I told him I wouldn’t be taken seriously,” Mr. Branch said. “Because I’m not objective about him.”