IF THE AGENCY was all that the publisher's publicists have cracked it up to be -- "filled with stories from a hitherto secret world" -- pressure would be mounting even as we speak for the presidential commission to confirm or deny these "hitherto" secrets. Congressional investigations would be cranking up. Sticklers for morality, government accountability, constitutional checks and balances, cherished values and the democratic way of life would be up in arms.

But, of course, we've had all that: The newspaper expose's; the "skeletons" laid bare; the handling over of the "family jewels"; the Rockefeller Commission; the Church Committee in the Senate and the Pike Committee in the House. We have been told more than we really want to know about poison pills and assassination plots, efforts to overthrow uncongenial foreign governments, the secret subsidies for friendly groups or institutions overseas. The interesting question now is whether the genie of covert operations can ever be stuffed back in the bottle, out of public sight and out of public mind. There lies the true value of this almost encyclopedic chronicle of America's relatively brief, often tortured and sometimes brilliantly successful experience with an agency of government entirely dedicated to what is loosely called "intelligence."

By meticulously tracing the CIA's history from its World War II origins to the present day, Ranelagh effectively defines the enduring conflict between the CIA's "secret world" and the open society it is supposed to serve. That he does not manage to resolve it is forgivable; the Reagan administration's bumbling efforts to put an end to breaches of security and wind the reel back to the days when "covert" meant what it says are reason enough to wonder whether it can be resolved.

Other books have traced the CIA's beginnings, going back to the wartime OSS, or focused on particular chapters in its history. There have been personal memoirs. My sense is, however, that there has been nothing as comprehensive as Ranelagh's detailed narration of the CIA story from the time of "Wild Bill" Donovan to today's William Casey ("Wild Bill" in his own fashion). Only those with first-hand familiarity as participants can vouch for its accuracy and no doubt plenty of nits will be picked. Nobody could get all the secrets straight in a bureaucracy whose cells are hermetically sealed even against each other. I speak to this point with some slim experience, having spent slightly less than a year in one of those cells some 35 years ago without venturing beyond the District of Columbia's first taxi zone on agency business. I left because I didn't even know what I was doing. So I cannot be sure whether Ranelagh has got it reasonably straight. Certainly the documentation is impressively voluminous.

But it is the total context that is most useful: the pictures presented before and after the whole thing blew up in the 1970s. That's what gives the sense of the enduring damage to public trust and the irretrievability of the past in practical political terms -- for all the Reagan administration's efforts to retrieve it. Whether we are talking about covert intelligence gathering or covert operations ("dirty tricks"), the experience of the CIA over roughly 40 years tells us that the key to passive public acceptance is public innocence. Since it is now impossible to retrieve that innocence, it is impossible, as well, to recreate the hot-house condition that allowed delicate, secret operations to flourish and encouraged the essential discipline and loyalty.

No such discipline exists today, as you may have noticed from your morning newspaper or the evening newscasts. A battalion of congressional overseers has replaced the corporal's guard of trusted congressional co-conspirators who used to keep a sympathetic eye on things in the intense times of the Cold War in the 1950s and 1960s. Supposedly "covert" activities are regularly disclosed or exposed. Congress openly argues over the merits of "covert" operations in Nicaragua, Afghanistan, Angola. Government officials openly solicit public approval of projects from which the government should be openly detached if they are to be effective. The notion of "plausible deniability," so dear to the dark world of clandestine operations for its diplomatic and political implications, makes no sense when the U.S. government is publicly revealing its hand.

You don't have to make your way through every one of the 847 pages of The Agency to get the point. But it helps, if only because the point is strengthened by the wealth of evidence. And it is hammered home by the quite different ordeals of two of the most interesting directors of central intelligence who were most directly caught up in a personal way in the fundamental, tormenting dilemma posed by the need to withhold from American citizens a knowledge of activities that responsible public representatives have judged to be vital to national security.

Both Richard Helms and William Colby had similar backgrounds as professionals in the intelligence business. Both served in the OSS. Yet each had developed a distinctly different view of the responsibilities of the same office. Helm's refusal to let the CIA get caught up in Richard Nixon's Watergate coverup reflected his feeling, Ranelagh writes, that the CIA "was responsible for itself first and the president second . . . its prime duty was to maintain the national security." In this spirit, Helms walked a fine line in congressional testimony about the CIA's role in Chile -- so fine a line that he was subsequently charged with perjury and eventually given a $2,000 fine and a two-year suspended jail term. The judge ruled that his oath to tell the truth took precedence over his oath of secrecy to the CIA. Helms saw his silence as essential to an effective intelligence service, and thus to national security.

Colby, whose voluminous disclosures of CIA secrets to congressional investigators led to the charges against Helms, took the view that the preservation of the CIA, once it came under public and congressional attack, required compliance with whatever demands Congress might make for access to the "family jewels." Old CIA hands will not forgive him. That the contrary positions taken by Helms and Colby were both arguable and honorable is no more than a measure of how difficult it is to reconcile national security demands with constitutional commands.

RANELAGH IS a British TV producer and investigative journalist whose two previous books deal with Irish history. This entitles him to a certain detachment in his conclusions. "In its beginning, the Agency was seen as doing hard things for hard reasons," he sums up: "Thirty years later it was seen as a contaminating agency. In the 1980s the nation does not want to be rid of it but is not proud of it." That's not a bad way of defining the difficulty the Reagan administration is having with public and congressional attitudes toward governmental secrecy and "covert" activities.

"What is crucial," said Secretary of State George Shultz the other day, "is the ability to take some initiatives quietly, in situations where the more the measures are known, the less effective will be their results. We have to get over the idea that covert is a dirty word." The Agency is a timely reminder of a great many reasons why the American people and their representatives in Congress are unlikely to respond positively any time soon to Shultz's almost plaintive -- and not unreasonable -- plea.