Shortly before he died in February 2007, Arthur M. Schlesinger Jr. — intellectual adviser to John F. Kennedy, leading exponent of 20th-century liberalism, and one of the greatest historians in American history — agreed to publish a sampling of the private journals he kept for most of his adult life. With his health failing, Schlesinger allowed his sons Stephen, a foreign policy scholar, and Andrew, a documentarian, to edit the 6,000 typescript pages. The resulting volume, titled “Journals, 1952–2000” (Penguin Press, 858 pages, $40) — lively, enthralling, surprising, witty — nonetheless will leave readers with a question: When can we read the other 5,000 pages?

More than a treasury, “Journals” is a treasure. You can open it anywhere, start reading, become instantly rapt, and suddenly notice that a morning has passed. To be sure, my interest in Schlesinger’s world may be especially avid: As a historian of American politics and letters with an interest in liberalism, I had the privilege of working with Schlesinger on a biography of Calvin Coolidge. Yet so delicious are Schlesinger’s anecdotes, so keen his observations, so glistening his prose, that any enthusiast of late-20th-century politics, regardless of political leanings, is likely to find the book irresistible.

Not that it hasn’t already elicited some jabs. Critics have extracted entries to charge that Schlesinger was too partisan in his opinions (the frequent lunches with Henry Kissinger notwithstanding) and too eager to cultivate the famous and powerful (these selections describe more parties than trips to the archives). These jabs are, of course, old news. They go back to the historian’s close association with J.F.K. — a subject on which the book, naturally, has much to say. Schlesinger’s commentary on Kennedy ranges from the sublime to the ridiculous. On one page he shrewdly analyzes the president’s strategic “habit of designating ‘liberals’ to do ‘conservative’ things, and vice versa.” On another, he gives a deadpan account of the swimming-pool high jinks at one of Robert Kennedy’s soirees. (Schlesinger’s dancing partner accidentally knocked Ethel Kennedy in the pool, while John Glenn, Byron White, and Harry Belafonte looked on.)

Yet however compelling these jottings about life in Camelot, even more fascinating are Schlesinger’s reflections on the imprint that his service to Kennedy made on his own later life. Sometime in the late 1960s, a nasty strain of Kennedy revisionism took hold, on both the right and the radical left. The entries in “Journals” show Schlesinger engaging frequently and vigorously with J.F.K.’s detractors, from biographer Seymour Hersh (“He is a fluent, impassive, unstoppable talker and hogged too much of the very short air time,” Schlesinger wrote of a joint TV appearance) to an unnamed journalist for the New Republic (“People tend to read back the current Soviet-American mood into 1961 and suppose that JFK was some kind of gratuitous and hectic Cold Warrior. … [But] the Cold War was … a hard and visible reality.”)

If Schlesinger seems at times defensive about Kennedy’s reputation — which, it should be noted, has largely withstood the denigrations — perhaps it’s because those denigrations often extended to Camelot’s chronicler as well. Schlesinger’s proximity to power — not to mention his astonishingly precocious success as a historian, marked by the publication of his landmark “Age of Jackson” when he was all of 28 — inevitably produced resentment.

During the febrile 1960s, for example — “the worst and saddest decade of one’s life,” Schlesinger notes, in a surprising and moving judgment, “the decade of the murder of hope” — some New Leftists venomously attacked Schlesinger, a New Deal liberal. One journal entry recounts a screening of Antonioni’s “Zabriskie Point” (“a hymn to violence and revolution, done in the most heavy-handed and simple-minded way”) at which a fellow theatergoer tore into Schlesinger — who moonlighted as a film critic for Show and other outlets — charging, “What are you doing watching this film? It’s about you and your kind and what we’re going to do to you. … We’re going to rub you out.” Meanwhile, William F. Buckley taunted Schlesinger in print as “the most overbearing liberal ideologue in the United States” and retailed fictitious stories about the historian, prompting Schlesinger to muse, “Whenever I encounter Buckley, he is excessively genial, as if he wanted to be friends; but, whenever I begin vaguely to soften under his personal courtship, something like this reminds me how odious he is.” As ever, Schlesinger occupied the vital center, fending off extremists on both flanks.

But if these journals sometimes show Schlesinger as partisan, effusive toward Kennedy, and socially omnivorous from an early age — “Lauren Bacall and Humphrey Bogart gave me a lift into town,” he notes less gleefully than matter-of-factly, in an entry from 1952, when he was just 35 — they also include unstinting words for some people he clearly holds in high esteem. He has critical words at times not just for J.F.K. — who he suspects is using John Kenneth Galbraith and himself to gain credibility with liberal intellectuals — but also for such admired pals as Adlai Stevenson (for failing to muster grace notes after Kennedy’s murder), Ted Sorensen (seen as overly proprietary about the speeches he wrote), and Ted Kennedy (who marvels uncomprehendingly that his late brother could enjoy the works of historians Samuel Flagg Bemis and Allan Nevins). It’s to Schlesinger’s credit that these entries come off not as dishy or petty but as measured; journals are, after all, the place where such thoughts ought to be recorded. It’s a credit, too, to the historian’s sons that they saw fit to publish the full range of their father’s judgments of his friends.

For all that, “Journals” is about much more than the Kennedys. Many of the most captivating passages come from the 1970s, 1980s, and 1990s, when Schlesinger was more removed — though never too far — from the center of the action. Consider a few.

On Richard Nixon: “I have never really doubted, since Watergate began to unravel, that Nixon would be removed or would resign before January 1977. This confidence was based essentially on a sense that Nixon was the greatest s— — probably the only s— — ever elected President of the United States.”

On Jimmy Carter: “In the end I could not bring myself to vote for a man who believes that Adam and Eve once existed and that Eve was literally made out of Adam’s rib (as he explained in a letter to the Atlanta Constitution) and believes he has seen flying saucers. So I left the presidential space blank.”

On Hillary Clinton and Joe Lieberman: “I made the point that the liberals had stood by Clinton [during the impeachment] while the DLC people had deserted him and described the miserable Lieberman as a ‘sanctimonious prick.’ Hillary said, ‘Well, he is certainly sanctimonious,’ but showed no eagerness to pursue this line of thought.”

One could go on. Each page contains gems. If the “Journals” reveal disappointingly little about Schlesinger’s personal life, and less still about the practice of writing history, neither are they an inconsequential romp through the social lives of the liberal elite. In evidence throughout is Schlesinger’s abiding concern for the health and direction of American democracy, as played out by the journalists, thinkers, and politicians who did so much to shape its course. From his passion for Stevenson to his outrage over Bush v. Gore, these diaries record, amid the frivolity and apercus, the dedication to a better society that has always animated men and women of a liberal temper — Arthur Schlesinger Jr. chief among them.

Mr. Greenberg, a historian at Rutgers University, is the author of “Nixon’s Shadow: The History of an Image” and “Calvin Coolidge.”