IT WAS MARK TWAIN who befriended the bankrupt Ulysses S. Grant and persuaded the former President to let him publish Grant's memoirs. The book made a fortune; American Presidents have been routinely selling their stories for huge sums ever since. Most of them are unmemorable and unrevealing - and unread - but they continue to adorn the nation's bookshelves as attractively bound dustcatchers.

Americans, of course, hold no monopoly on producing memoirs. The practice of celebrating the Great Leader's Life by putting pen to paper (or hiring a ghost) dates back at least to Caesar's Commentaries. But it remains for Richard Nixon, fittingly if ironically, to break new ground in the genre of personally recalling past powers and glories. Fitting because Nixon, more than any modern public figure, has been absorbed the longest in studying the techniques of mass communications, of everything from polls to PR. Ironic because, unlike other leaders, what makes his story most salable are his recollections of his disgrace, not his triumphs.

Richard Nixon has now ushered in the era of the big-time, big-money, electronic memoir, packaged, produced and prepared, with appropriate commercial interruptions, for television.

Nixon's not the first to cash in big on the trend of public figures drifting into television. Lyndon Johnson sold his version of the presidency to CBS, John V. Lindsay has become a TV news personality and Gerald Ford and Henry Kissinger have signed contracts to become news commentators for NBC. But Nixon's million-dollar deal with David Frost is bound to set the standard for future personal political TV productions. None before his attracted the fanfare, the advance magazine cover stories, the front-page banner headlines, the sense of new and dramatic disclosures to be made. And none yielded the continuing exposure, and recriminations and reactions from the present President on down, which we have witnessed these last four weeks.

It matters not that there were virtually no new disclosures, no new areas explored, nothing of hard substantive difference developed, nothing that alters, in any way, the conditions of the case against the 37th President of the United States.It matters not that troubling questions about "checkbook journalism" are being raised. Nothing will be done to change them, anyway.

What matters, in terms of the future, is that The Return of Richard Nixon, the lastest installment of the longest-running spectacle in American political history, was a television success. It made money; it drew the mass audience; it will be emulated. What sold was the shame of the presidency.

NIXON, IN PARTICULAR, was the perfect subject for such a vehicle. He's been around longer, and as stirred more controversy, than any politician in our story. No one has been dissected, by himself and by [WORD ILLEGIBLE] more intensively and compulsively for year after year, decade after decade. yet our appetites for information about him obviously remain ravenous.

Part of this, surely, has to do with the circumstances his fall and his subsequent exile. His near fatal illness and withdrawal into brooding reclusion at Casa Pacifica, is splendid estate in San Clemente, inspired reports of a personal breakdown, of disturbing evidence of mental and physical disarray, of untended lawns, of leaves filling the empty swimming pool. All this, combined with evidance reports of Nixon being ruthlessly interrogated and trapped into damning admissions, pandered to our most prurient instincts.

What did Lear really look like after the storm? What has Napoleon really like on Elba?

After all the buildup, the Nixon we saw was the Nixon we knew. If anything, he looks far better than in his presidential days. More weight and more gray becomes [WORD ILLEGIBLE] he appears less drawn, less haggard and, remarkably, less tense. And the Nixon we heard was the old Nixon, employing all the old devices familiar for a generation.

You can read over the four transcripts of the Nixon-most interviews and find almost nothing of fact that hasn't known before. Frost stuck to the record, and seldom ventured into unexplored territory. How did Nixon, for instance, know where to raise a million dollars in cash, quickly and easily? Had he dealt with such amounts before? How and why and for what purpose? Why in cash? Any why did his good friend Bebe Rebozo, banker by profession, maintain huge sums in cash for years at a time? Why didn't Nixon destroy the tapes that candid him, and what were they for anyway? What about the role of the CIA, alluded to but never really explored on severeal occasions? What about assassination plots? What about the 18 1/2-minute gap?

THE QUESTIONS are endless, and maybe in the end they are incidental. For with all the flaws, the Nixon interviews were extraordinarily revealing. And far more so than ever could have been compressed in the pages of a newspaper, magazine or book.

Television is the most subjective medium. It enables us all to respond to, or react against, the subject of the camera in the most intimate and personal way.

Aside from financial considerations, Richard Nixon clearly looked upon the interviews as a personal opportunity. Whether he saw them as a chance to rehabilitate, to resurrect, to repatriate himself with his fellow Americans can't be answered. But at least he hoped they would restore his reputation. That now highly unlikely.

It could well turn out to be that the picture we saw of Richard Nixon on successive weeks in the month of May resolved, once and for all, any lingering doubts about his character. The Nixon who entered our living rooms was incapable of conceding guilt, incapable of generosity, incapable of telling the truth. He seemed a stunted, tragic figure, turning on almost everyone who has been closely associated with him.

Eisenhower couldn't bring himself to deal with Sherman Adams; it was Nixon who had to do the dirty deed (a version, one of many, that has already been denounced as being untrue). Kissinger, whom he praises lavishly when it helps, as in the final praying-crying scene, was cut down as egotistical, power-hungry, celebrity-conscious, given to erratic, if not unstable, switches in mood and tone. Haldeman and Ehrlichman, his loyal aides, his two right arms that he was forced, unwillingly, to amputate, were consigned to guilt and final blame by their former chief. The same with Mitchell, and almost everyone else who crosses the fallen president's path - his lawyer, his attorney general, his White House aides and on and on and on.

Watching Nixon twist the record, and turn on associates, one has the uncomfortable feeling he really believes what he says. His latest version is the correct version; what helps at the moment is the ultimate truth.

Another side of his character comes through with painful clarity. When his secret White House conversations were made public, many expressed shock at the language and cast of mind they revealed. Years later now, the Frost interviews expose the same ugly-spirited tone. He speaks of critics as "punks" and "trash," of wanting to "crack'em in the puss."

TO THIS WRITER, the two most revealing episodes of the interviews had nothing to do with Watergate. The first dealt with Spiro Agnew, the second with Chile.

In discussing the Agnew case, Nixon at no point expresses any sense of moral or legal concern about the historic criminality the government had uncovered. He reacted to the news, he recalled, pragmatically. In fact, nowhere in the four interviews is there the slightest hint that any of the assorted wrongdoing elicited a spark of outrage from him. When he talks about Agnew he defends him as an "honest man," in his heart "a decent man," and approvingly observes:

"I think that he felt that he was just part of a system that had been going on for years, and that it was accepted in the state that people who did business with the state would help the governor out with expenses that he might have that he couldn't take care of out of his own salary."

Everybody does it!

This, from the lawyer-turned-politician who campaigned on public probity and integrity and against criminality, and whose domestic presidential theme was restoring law and order.

The questioning on Chile was revealing for a different reason. Richard Nixon's best hopes for a kinder future judgment rest primarily on his foreign policy ventures. He is praised particularly for his role as a world statesman bringing about the beginnings of rapprochement with the old Cold War nemeses, the big Communist states of Russia and China.

When David Frost challenges Nixon on his evaluation of the dangers of Chile ot the hemisphere, Nixon responds heatedly and repeatedly by retreating into his past. His answers, again and again, sound the themes of Dick Nixon, the militant anti-COmmunist of the late 1940s and early 1950s.

"It shows somebody cutting through the hypocritical double standard of those who can seel all the dangers on the right and don't look at the dangers on the left," he says, citing the combination of little Chile and Cuba as a "red sandwich" imperiling the United States.

This, from the polished diplomatic negotiator with Mao and Brezhnev whose foreign presidential themes heralded a new era leading away from the ideological confrontations of the past.

Obviously, frou televsion interviews do not a definitive portrait make. And, obviously, we will hear more from and about Richard Nixon. But a case can now be made that we have seen as much of the "real" Nixon as we are likely to be shown. Or should even want to see. If the picture builds sympathy for his human failing and anguish, but lessens personal respect, it would be an unhappily faithful rendering. And we have seen something else, something even sadder.

We have seen a President of the United States be charged with high crimes and misdemeanors, resign, cop a pardon, live on a handsome lifelong government pension still surrounded by Secret Service agents and still receiving top-secret presidential briefings, sign a TV contract, go before the nation, confess to misguided motivations, difine a President's powers as being above the law and then take home a million.