ALL FALL DOWN; America's Tragic Encounter With Iran. By Gary Sick. Random House. 366 pp. $19.95; AMERICAN HOSTAGES IN IRAN. The Conduct of a Crisis. Warren Christopher et al. Yale University Press. 443 pp. $25.

WHEN JIMMY CARTER's ambassador to Tehran, William H. Sullivan, wanted to shock the bureaucracy into responding to his Nov. 9, 1978, cable reporting that Shah Mohammad Reza Pahlavi was losing his grip on power, he entitled his report: "Thinking the Unthinkable." In fact, Sullivan thought the predictable. He suggested the United States begin to prepare for the emergence of secular politicians, or army generals, as the successors to the shah. Iran would become "Kuwait writ large," and the Ayatollah Ruhollah Khomeini would return to Iran to hold a "Gandhi-like position."

As we know now, Iran came under the control of neither the pro-Western secular politicians that Sullivan hoped would salvage the situation nor the pro-Moscow Tudeh party that Washington feared as the worst possible outcome. Instead, fanatical Moslem clergymen seized power, encouraged a mob of youths to hold an entire U.S. embassy hostage for more than a year, destroyed Carter's presidency and helped pave the way for an unopposed Soviet invasion of Afghanistan.

It was an outcome the system could not anticipate, in part because policy making in Washington tends to be dangerously incremental. The bureaucracy works to reduce risks for policy makers by matching predicted outcomes to available responses. Jimmy Carter could have responded successfully to the prospect of Iran becoming Kuwait writ large -- that is, a Moslem country with powers split between a ruling family and an active, democratically elected parliament. An adviser who in 1978 urged him to prepare to sed troops to Iran or accept the real disasters to come would have been quickly outmaneuvered within the policy- making establishment.

For Americans, Iran was a failure of imagination as much as it was a failure of intelligence-gathering, nerve or power. At each stage of the separate crises of 1978-1981, officials of the Carter administration (and academics, journalists and others dealing with events there) could not free themselves from a system that kept their hypotheses, and suggested responses, confined to the level of the manageable.

In the torrent of books that have examined the fall of the shah and the hostage crisis that followed, that failure has not been fully addressed. The principals of the Carter administration have argued their roles in their memoirs, the hostages have told us their dramatic stories, and analysts have sought to expose failures of intelligence agencies, of political will or of American culture as the chief culprits. But key pieces of the puzzle of our incomprehension have remained cloaked in shadows.

NOW, two new "insider" books provide valuable insights into the inability of our government to anticipate events in Iran or to understand them once they had occurred. Gary Sick's All Fall Down does so knowingly, in a compelling narrative that moves a general reader through the maze of policy making. It may be the single most important account we will get on the American policy failures in Iran.

American Hostages in Iran on the other hand is written by specialists for a specialist audience. It is a collection of essays written by the members of the negotiating team that finally succeeded in obtaining the release of the American hostages and constitutes an unusual case study in the practice of diplomacy and negotiations. But the authors too often appear concerned with justifying rather than examining their decisions, and the political judgments of some of the senior policy makers in the Iranian crisis display inadvertently the reasons why we are still plagued by the question of how we could have lost all influence over a country we defined as vital to our interests in a matter of months.

The bloodless, dry decision-making in Washington that excluded an understanding of the passions that had been unleashed in Iran is exemplified perhaps by this judgment from Warren Christopher, who was Cyrus Vance's deputy secretary of state and head of the negotiating team that arranged the hostages' release. Christopher, after seven years of chaos and repression in Iran, apparently still believes the following:

"There is a school of thought which holds that, given the mindset in Iran, the difficulty and anguish caused to the United States and its total estrangement from Iran were in themselves an achievement. In this view, the isolation created and the inability of the Americans to find or force a solution for fourteen long months gave the Iranians a psychic satisfaction that outweighed the tangible disadvantages. . . . In view of the Iranian obsession about 'the Great Satan,' one cannot completely dismiss this viewpoint; still, I doubt that any nation would find such a psychic ride to be enough compensation for the massive losses Iran suffered."

Well, the Iran of the mullahs clearly did, and Gary Sick is much closer to the mark when he observes in All Fall Down that the hostage crisis was "a constitutional crisis played out in terms of national psychodrama." Christopher's argument notwithstanding, it should be clear by now that the unending search by our diplomats for rational norms within the Iranian leadership that they could manipulate into a solution was a wasteful and dangerous exercise.

Despite the flaws of some of the analysis of Iran and the Third World in American Hostages in Iran, official Washington should read carefully the description of the structural problems in policy making that Christopher provides in his introduction to this Council on Foreign Relations book. Christopher describes the trap that the formal structure of the National Security Council became for Carter at crucial phases of the hostage crisis, and makes suggestions that any White House will find useful.

Gary Sick, a retired naval captain who served as Zbigniew Brzezinski's principal aide on Iran on the National Security Council staff in the Carter White House, has written a largely dispassionate account that makes heavy use of previously classified documents and notes and Sick's own keen eye for telling detail. Sick was often the notetaker at the crisis meetings on Iran, and he developed the deepest, most sustained sense of the American response to the Iranian challenge to Carter's presidency.

He gives us new perspective on key events such as Sullivan's crucial November cable and his doomed efforts later to negotiate a transfer of power to the pro-Western politicians on behalf of the United States without telling Washington what he was doing until "the very last moment as the situation became desperate, almost as a fait accompli" and far too late to be effective.

TO UNDERSTAND better our continuing failure in Iran after we had been bitten once by the shah's downfall, it is useful to compare Sick's description of the creeping enlargement of the U.S. embassy staff in Tehran in 1979 after it had been cut back in wake of the February takeover with the justifications of business as usual presented by the policy makers for that process in American Hostages in Iran.

Here is Sick: "The governing attitude was to encourage normalization and therefore to permit the embassy to increase gradually in size. . . . The problem of bureaucratic 'creep' . . . normally occurs at the working level where it is essentially invisible to the policy maker. . . . More difficult to explain is the proliferation of files." In a mindless reflex, agencies in Washington shipped back to Tehran classified documents that had been sent out of the country during the February siege. (Although Sick minimizes the damage done to national security by the capture of these documents, the result has been that the mob that took the embassy that November is now publishing a body of classified U.S. information that already surpasses the Pentagon papers in disclosing secrets.)

Or take Sick's own description of the memo he wrote to help persuade first Brzezinski and then Carter to mount the ill- fated hostage rescue mission. He portrays the rescue mission as the bureaucracy's perfect option, bringing the highest payoff if it worked, and with the lowest risks if it did not. The mission was planned so that it could be cancelled at any point along the way. Given the enormous odds against the mission, it is revealing that there appears to have been no discussion of a strategy that would have absorbed the disaster that did happen -- the unthinkable -- into a next step to improve, or resolve, the crisis.

I am impressed by Sick's provocative observations about the performance of the American military in the rescue mission. It failed in large part, Sick argues, because "human judgment was decisively influenced -- even overriden -- by technology. . . . In two critical cases, when machines failed to operate as anticipated, the mission was abandoned." He refers to decisions by two helicopter pilots to abandon the mission and thus force its cancellation because of instrument failures that would almost certainly not have endangered them. "As members of the premier technological culture, we have been trained from infancy to heed and even to subordinate ourselves to machines."

Unfortunately, Sick seems uncomfortable with making this kind of broad judgment and too often leaves the reader to draw the larger meanings from the events and facts he recounts. A final chapter looking back over and analyzing the points he has made along the way would have been worth the effort and would have almost certainly enhanced this already valuable book.