ONCE UPON a time, The Washington Post suffered from poor editing and slanted reporting on the side of liberal causes, encouraged by City Editor Ben Gilbert. Then it suffered because Philip L. Graham, the publisher, lusted for political power, which caused people to say that The Post was too liberal or too Republican or too much in the pocket of the CIA.

But Graham died, and Katharine, his widow, hired Ben Bradlee as editor, and although he swore a lot and wore loud suits, Bradlee hired a lot of good reporters and got them all hopped up to do wonderful things like driving President Nixon out of office.

But Watergate made them arrogant, Sally Quinn returned from CBS, and Janet Cooke won the Pulitzer Prize for a fictitious story about a child junkie, and the gossip column "Ear" falsely reported that Jimmy Carter had wiretapped Ronald and Nancy Reagan, and all this produced a loss of credibility, which is the worst thing that can happen to a newspaper except maybe a loss of money.

That's the plot of The Imperial Post, an entertainment written by Tom Kelly, a Washington free lance, and endorsed (on the jacket) by Dick Tuck, "political commentator."

One thing you can learn from this book is who really runs The Washington Post.

Sally Quinn, that's who.

"The Post grew increasingly reckless and arrogant," Kelly writes. "It seemed to acquire a new set of guidelines that could have been enunciated by Sally Quinn: pick a proper victim, then gather, select and discard evidence to support your intended conclusion . . . She set the style and no one said no. She was Bradlee's girl friend and then his wife and to her Bradlee's old rules did not seem to apply."

The Imperial Post might well run as a serial in People magazine. Everything happens because someone likes someone or hates someone, is afraid of someone or is jealous of someone. It's gossip.

Sally Quinn's adventures, including her foray into the world of television, cover 14 pages, far more than is allotted, for example, to Donald Graham, the new publisher of The Post. Next best subject for gossip is Agnes E. Meyer, Katharine Graham's mother, who kept "falling in love" with people--men--with results that Kelly does not specify, except to say that her husband, Eugene, was annoyed.

Kelly's description of the nervous, competitive atmosphere of The Post's newsroom is convincing, and he offers seemingly valid evidence of lapses from reportorial standards, particularly in the pages ot The Post's "Style" section.

The book is highly readable if you can put up with the style, especially the irritating use of the "false future" tense--the "he would" construction as a replacement for the past tense: "Phil would be disappointed"; "Agnes would continue to write for The Post . . ."; "Ben Gilbert . . . would be the next to be bumped." He predicts things that have already happened.

The text includes anonymous quotes by present or former Post employes, more of them than would have seemed necessary. Kelly says that an atmosphere of fear pervades The Post, and this may be true, for after a list of those he had "spoken with" during his researches he appends a note: "There are others who would prefer not to be named."

Even more annoying is Kelly's practice of quoting people without indicating when or under what circumstances the comments were made, or from which books or magazine article they might have been lifted. The absence of such information makes a reader confused and suspicious.

"Katharine would say later," Kelly writes, introducing an excerpted comment. Excerpted from what? How much later?

Kelly writes in short, punchy sentences, and even in short punchy paragraphs.

"Sally was listening."

"Mitchell exploded."

Some people may like that kind of writing.

Kelly's research seems to have been concentrated on previously published material; he is a synthesizer rather than an investigator, and his reports are naughty rather than scandalous. He is careful, and lawyers, I suspect, were among his editors. He has, however, amassed an impressive collection of fatuous comments, errors in judgment, examples of bad taste and sneaky photographs.

And this collection does contribute to an understanding of The Washington Post. The Post has been an important, established, prosperous paper for 30 years, but it has also been an enigma. It has never found its real identity or established its true character. It has employed talented, often brilliant professionals who have never managed to turn out a thoroughly professional newspaper.

Perhaps this was because neither Eugene Meyer nor Philip Graham, the first two publishers of the modern Post, came from a newspaper tradition. Neither seems to have known what to do with the newspaper. Katharine Graham, however, grew up with The Post and possessed a clearer sense of purpose. Yet she is the one character in Kelly's book who does not emerge as a real person. She seems full of contradictory qualities, sometimes imperious, sometimes humble; shy and rude, impulsive but coldly calculating; dignified but capable of vulgarisms.

Is it too imaginative to speculate that The Post reflects these same ambiguities?

The Washington Post chapter in Martin Walker's Powers of the Press ends on the same note as Kelly's book, with the loss of credibility or, as Walker puts it, "its failure as a reliable newspaper." He also cites the Cooke and "Ear" embarrassments, and quotes The New York Times: "When a reputable newspaper lies, it poisons the community. Every newspaper story becomes suspect." He agrees with Kelly that The Post has fallen short of greatness.

Walker's history of the The Post, in 20 pages, is meatier than Kelly's, and includes the information that The Post had vigorously opposed the anti-communist crusade of Senator Joseph R. McCarthy, a point that Kelly failed to mention.

Walker, a reporter for the Manchester Guardian, has chosen to describe the 12 most "influential" newspapers, not necessarily the 12 best. These are The Times of London, Le Monde, Die Welt, Corriere della Sera, Pravda, Al-Ahram, Asahi Shimbun, The New York Times, The Washington Post, The Toronto Globe and Mail, The Melbourne Age and The Rand Daily Mail.

Walker shows a keen appreciation of American press freedom. "American newspapers are in a uniquely enviable position," he writes. "The First Amendment to the Constitution says bluntly that Congress shall make no law abridging the freedom of the press." He gives The New York Times credit for leading the resistance to government efforts to subvert this provision.

"When Abe Rosenthal (The Times' executive editor) says that his role is 'to keep The Times straight,' he understates his case," Walker says. "The role of the American press, led by The Times, has been to keep America straight, to add an independent set of teeth to those freedoms which successive American governments and Intelligence services tried to undermine, in defiance of the Constitution."

The Anglo-American tradition of press freedom is the result of economic privilege and political stability and the concept of "the Law" as a historic constitutional entity in its own right, independent of government, Walker says. But in any nation where a balance of powers is maintained between the executive, the legislature and the judiciary, the press will develop a tradition of freedom "in the tensions and cracks between them, just as grass grows between paving stones."

The task of compressing a century or more of newspaper history into 20 pages necessarily creates some over- simplification, but Walker manages to convey a great deal of information and an understanding of the basic character of the publications. His conclusions are that important newspapers are run by the same kind of elitists who run governments, and that while the newspapers reflect the countries' Establishments, they act as a check upon the governments as well.