When an author goes probing for classified government documents

A little over a year ago, Stanley Kutler learned of a letter written in March 1943 by J. Edgar Hoover. The then-FBI director wrote to President Franklin D. Roosevelt about dissident labor leader Harry Bridges, who was under investigation by the government for alleged treason.

Professor Kutler - a constitutional historian from the University of Wisconsin - requested a copy of this ''classified'' correspondence under the Freedom of Information Act (FOIA). He explained it was needed for a book he was researching on the US government's pursuit of communists and suspected subversives during the cold war.

The FBI turned over the letter. But the entire body of it was deleted by magic marker. However, the researcher then contacted the FDR Library, which willingly complied with his request for a service charge of $5. The letter merely informed the President of a planned rally in behalf of Bridges.

Why was this information, almost 40 years old, withheld? Professor Kutler asked the FBI - but got no reply. However, the historian sees no sinister plot here to deprive the public of its right to know. His recent experiences with a multitude of government agencies from whom he has been drawing bits of information for his manuscript indicate that they are not so much devious and uncooperative about disclosing information as ''disorganized'' and caught up in bureaucratic red tape.

Kutler's book ''The American Inquisition: Justice and Injustice in the Cold War'' (New York: Hill and Wang. 285 pp. $16.50) came off the presses the day before Thanksgiving. It's a fascinating compilation of selected case studies of political prosecutions, or persecutions, during the post-World War II days of McCarthyism and anti-communist hysteria.

The constitutionalist painstakingly, but very dramatically and humanly, tracks through the pursuit and prosecution of Iva Ikuko Togura d'Aquino (Tokyo Rose); the blacklisting of Beatrice Braude from government service on the basis of flimsy evidence of association with a known communist (Judith Coplon); the long but fruitless probe of leftist labor leader Bridges on charges of subversion; the investigation and denounciation of Far East scholar Owen Lattimore by Sen. Joseph R. McCarthy; and the curious case of poet and literary giant Ezra Pound - indicted for treason but never brought to trial and tucked away in a mental asylum until the conclusion of World War II.

Kutler's published work carries a variety of messages. Among them: The rule of law must be followed at all times, regardless of the internal or external perils; prosecution for political or propaganda purposes is illegal under the United States Constitution; lawyers who defend communists or others in public disrepute must be protected from political harassment or worse; and the public and the press must guard themselves from getting emotionally involved in and manipulated by the events of the times.

In a Monitor interview, Professor Kutler insisted that, despite temporary lapses of justice and government pursuit of individuals for ''political'' purposes, the ''rule of law'' does work.

In the cases cited, among others, the course of administrative processes eventually led to the pardon of Mrs. d'Aquino (by President Ford on his last day in office); the dismissal of charges against Bridges and Lattimore; and vindication of Miss Braude.

Each took time - often many years - with personal tragedy, financial loss, and public disdain and repudiation as the price. ''But it was our institutions that saved us'' says Kutler. ''A court here, a court there, an agency here and there which brought things back to some semblemce of sanity.

''If we don't have a rule of law - for the ruled and rulers alike - the alternatives would be unthinkable,'' warns the constitutional scholar.

Perhaps significantly, it was the rule of law which worked rather well, if not always speedily, that allowed Professor Kutler to succeed in his research. He requested and received hundreds of pieces of data - official documents, letters, memos, reports - from the government under the Freedom of Information Act. And this led to new findings as well as verification of material obtained elsewhere.

For example, Kutler says he was the first person to get to see the medical files of Ezra Pound, which were sequestered until recently. These tended to back up the researcher's theory that ''Pound was as sane as could be . . . that he devised this (insanity) defense for himself because he was afraid of being convicted and concerned for his own literary reputation.'' Kutler contends that government officials were aware of this ploy and willingly conspired with Pound because he was a famous figure and a trial for treason would have been ''politically'' undesirable.

Of FOIA, Kutler stresses that the public should be aware that ''there are many acts and many officials who administer them.'' Some departments more willingly part with information. (''The military, for instance,'' says Kutler. ''Are you surprised? Well, they're used to following orders.'').

''The Central Intelligence Agency gives you nothing unless they want to,'' says the author. ''And Justice - they're the worst. But that's partly because they are enormously understaffed.''

Kutler adds that he considered adding a chapter to his book: ''My Adventures Under FOIA.'' ''But that might be better left to a humor magazine,'' he jests.

''We might be better off with just a simple 30-year-rule like the British have,'' Kutler suggests. ''Just give it to us after 30 years.''

Might the US again be vulnerable to the hysteria and the resulting ''political'' prosecutions of the 1950s?

Professor Kutler is optimistic that an enlightened public and press today are far less likely to fall prey to this sort of thing. He concedes that, although the fear of communism is still very strong, there is no particular concern about internal subversion now, as there was three decades ago.

''For the United States in the cold war era,'' the author explains in his epilogue, ''it was the worst of times. . . . There was a large cast of villains and victims; still it was not 'Darkness at Noon.' It was not a closed system.

''We have a society that permits the individual to resist under the rule of law and to use it to rectify the violations of the norms of that system.''