Armando Valladares, a Cuban poet, for 22 years a political prisoner in Castro's jails, has written the shocker of the year, "Against All Hope: The Prison Memoirs of Armando Valladares." He depicts Fidel Castro as a kind of Latin Stalin, a monster of treachery and cruelty.

Several questions haunt the sickened reader of these pages. We are by now hardened to prison memoirs. They have a sickening sameness, the same sadistic guards, the same hellish abuse that drives some prisoners mad; and others, like Valladares, to stubborn resistance. He refused during his entire stay, to submit to political rehabilitation or to wear the uniform of a common criminal. As a minor bureaucrat in Havana, he committed the crime of protesting communism.

Why didn't the Cubans kill him? Death was a common, casual thing in the prisons he describes. Beatings with electric cables, bayonettings and eventually biological experimentation were routine. His jailers succeeded in crippling him temporarily. They forbore to finish him off, even when his wife's intervention with foreign governments caused acute official embarrassment to a regime that claims it has no political prisoners.

The other question is, why didn't we know? Castro jauntily denies all human rights problems. He also bars all human rights visitations. The right wing blames left-wing naivete about Castro, the voluble, mocking prankster-dictator who hugs his visitors and talks them under the table.

In Chile, for instance, when the repression began, human rights groups here were almost instantly informed. One reason is that the Roman Catholic church organized the resistance. The church in Cuba never did. Valladares says that reprisals for the slightest protest -- such as writing a slogan on a wall -- are so savage that there can be no organized resistance.

It is more than curious that the Reagan administration, with its obsession about containing Castroism, particularly in Nicaragua, has been so silent about these freedom fighters, who up to now had no voice.

Maybe it is that the Reagan record on Cuban political prisoners is so lamentable. Wayne Smith, who was in charge of the Cuban Special Interests Section during the Carter years, points out that the Reagan administration has failed to admit about 2,000 political prisoners whose release, on condition that the U.S. would take them in, was being negotiated in 1978. After Reagan was inaugurated in 1981, no official yes or no was ever given. The prisoners are still in Cuba.

The State Department says that all admissions are being delayed because of Castro's rage. When Radio Marti, a special U.S. program, was beamed to Cuba, he canceled the immigration accord that was finally reached in December l984. The agreement was based on a Cuban promise to take back the 2,000 "excludables," the criminals dumped on us by Castro at the time of the Mariel exodus.

But the political prisoners, as pointed out by such a friend of the administration as Michael Ledeen of Georgetown, can be "paroled" into the country by the attorney general without regard to the accords. Valladares said grimly, "You are not punishing Castro by keeping these people out, you are punishing his enemies."

Valladares owes his freedom to French President Francois Mitterrand who, responding to agitation by international PEN clubs in France and Sweden, made a personal appeal to Castro for Valladares' release.

Valladares is a compelling person. His slight frame gives no hint of the iron will that kept him going even when forced at bayonet point to go into a ditch filled with human excrement. He has a surprisingly youthful face, and he talks in torrential Spanish about the "companeros" who still suffer in the gulag he escaped. He is not making the standard literary promotion tour. He is on a crusade for their liberation.

With him is his wife, Martha, a warm and gentle creature whom he met when she came as a teen-ager to visit her father in prison. When she left Cuba on a freedom flight in 1972, she began traveling the world to broadcast the horrors of he wrote to her on scraps of paper smuggled out of jail. She waited for him 22 years -- longer, he said, "than Penelope waited for Ulysses."

Valladares says what sustained him in his long agony was his belief in God, his resolve to love rather than to hate, and "fantasies about Martha and the children who had not yet arrived." They now live in Madrid with their two sons.

Some reviewers and human rights experts express doubts about Valladares' reliability. They think he has attributed to himself experiences of others. It is not important. If a fraction of what he recounts in "Against All Hope" is true, we have abounding evidence that Castro is steeped in blood and that attention, at last, must be paid to his most pathetic victims.