The American people can feel reassured that President Reagan has unequivocally divorced himself from any knowledge or use of a ''briefing book'' obtained from the Carter White House for a presidential debate in 1980. It is also reassuring that he has instructed the Justice Department to determine how such politically advantageous material fell into the hands of his campaign aides and, in his words, ''to take whatever action is appropriate'' if something ''improper or illegal'' took place. Without the full facts of the situation, a judgment in the matter is premature.
However, this is not to hide a certain uneasiness many Americans will feel at this point. A good share of Mr. Reagan's press conference this week was taken up by questions about the ethics of accepting and using possibly purloined material. The President might have turned off such queries by declaring at once his strong disapproval of any unethical conduct by his aides and his determination to make certain it is not a part of his administration. When pressed, he did say that ''politics should be above reproach'' and ''there shouldn't be unethical things done in campaigns.'' But the impression left was of a President cautiously avoiding direct answers and blaming reporters and Democrats for blowing up the affair for political motives.
The issue is not whether Mr. Reagan won in 1980 because of help received in that critical Ohio debate. No one suggests that is the case - and, least of all, Democratic stalwart Tip O'Neill. It ism a matter of the moral and ethical tone set in the presidency and of what standards should be expected from America's leaders and politicians. Too easily it can be said that dirty politics - including ''spying'' - is the norm and that ''everybody does it.'' Perhaps so - and perhaps not. Congressman Morris Udall was offered material from the ''enemy camp'' in the course of a campaign and simply refused to accept it.
But, if political shenanigans are often the norm, is that a norm to be apathetically tolerated? Should there not be voices - loud and clear - resisting the temptation to settle for less than integrity in the conduct of campaigns as well as in office? It is no wonder that public cynicism about government is widespread. Surely after the searing experience of Watergate public leaders at every level have a responsibility to help dispel that cynicism - to make politics clean and honest and government the respected institution it should be. If ethical questions are dodged, what can the public think?
Press and public should take care not to exaggerate the present case, however. There is nothing to suggest that the briefing book affair is a Watergate, and summer doldrums should not induce news media to overplay the subject. Needed now is a calm, dispassionate, thorough investigation of the questions raised: Was the material passed along by a disgruntled Carter worker, as the President suggested? Or did Reagan strategists plant a ''mole'' in the Carter establishment? What was the role of CIA director William Casey, who was Mr. Reagan's campaign manager at the time but who denies any knowledge of the material? How did the White House obtain the papers from the Carter campaign which it found in the files of current aides and released this week?
There can be no political cheer - and there should certainly be no titillation - in the fact that the White House is again touched by potential scandal. Wrongdoing, wherever found, affects all of society. The integrity of the Reagan presidency will not rest alone on what may or may not have been done in the 1980 campaign, but on the probity and courage displayed in dealing with it. The public expects these to be ''above reproach.''