In April 1959, just months after a charismatic 32-year-old revolutionary named Fidel Castro seized the reins of power in Cuba, a slim volume of his correspondence, titled "Cartas del Presidio," or "Letters from Prison," was published in Havana. The book contained 21 letters addressed to Castro's inner circle of supporters, including his wife, Mirta Diaz-Balart; his half-sister, Lidia; a future mistress; the father of a fallen comrade; and nine missives to his devoted friend and political devotee, Luis Conte Aguero, who published the book.

The letters, however, have not appeared in English until now, and after 1960, when a disillusioned Conte Aguero fled Cuba, no further copies were printed in Havana. Nevertheless, this collection of Castro's writings -- virtually the only unofficial writing he ever did -- has become something of a Rosetta Stone for historians, biographers and journalists seeking to understand the man who would become Cuba's ruler for life. Some may argue that a careful reading of the letters foretells what would transpire in Cuba over the next 50 years. Others could say that the Castro of these letters is not the Castro he would become.

Both are true to varying degrees.

The letters begin several months after Castro's ill-fated attack on the Moncada military garrison. The brazen assault made Castro a household name. But it also irreparably wounded his wife's family, which included ministers in Fulgencio Batista's cabinet. Castro was now directly at war with his brother-in-law, deputy interior minister Rafael Diaz-Balart, who had introduced Fidel to Mirta when the two men were friends at the University of Havana.

To supplement her income while Castro was in prison, Mirta accepted a modest stipend from her well-connected brother through his government ministry. Castro's discovery of this arrangement catapulted him into a bottomless rage. Ruled by pride, as he has been throughout his life, he perceived the bequest as an attack on his honor -- never mind Mirta's needs.

In a letter to Conte Aguero, he railed:

"I never imagined that Rafael could be such a scoundrel and that he had become so corrupted; I cannot conceive how he could have so pitilessly sacrificed the honor and name of his sister, exposing her to eternal shame and humiliation. . . . It is a chore to push away the mortal hatreds that seek to invade my heart. I do not know if there is anyone who has suffered more in these past days. It has been a terrible and decisive test, with the capacity of quashing the last atom of kindness and purity in my soul, but I have made a pledge to myself to persevere until death. . . . After such weeping and sweating of blood, what is left for one to learn in the school of sorrow?"

Castro and Mirta divorced while he was in prison, and she remarried and moved to Madrid. Although they reconciled about 10 years ago and the widowed Mirta returned to live in Cuba at Castro's request when he became ill, Castro waged a scorched-earth campaign in 1955 to keep his son Fidelito. Custody of his son -- and of his country -- became his twin jailhouse obsessions. And his battle for both reveals a man with an indomitable will and steely determination. Of his fight for Fidelito (which he won) he wrote to his sister Lidia: "I do not care one bit if this battle drags on till the end of the world. If they think they can exhaust my patience and, based on this, that I am going to concede -- they are going to find that I am wrapped in Buddhist tranquility and am prepared to reenact the famous Hundred Years War -- and win it! To these private matters, add my reflection on the political panorama -- and it will not be difficult to imagine that I will leave this prison as the man of iron."

The letters amply illustrate Castro's many gifts: his formidable erudition, strategic thinking and natural leadership. They are also an early indicator of his Machiavellian cunning and his genius for public relations. And they dramatize his resentments and rages. Castro was remorseless and unforgiving of his perceived enemies, a man for whom compromise was a mark of weakness. In another letter to Lidia, he boasted, "I have a heart of steel and I will be dignified till the last day of my life." What must this intensely proud and private man have felt about the public disclosures of his recent medical travails, in which every inch of his intestines has become fodder for the world media?

In an early letter, from December 1953, Castro decides that he and his followers will forgo Christmas as a protest against authorities. "It is decided we shall not have Christmas -- not to even drink water on that day as a sign of mourning. . . . There is no point for prisoners like us to aspire to the joys of Christmas." Castro banned the public celebration of Christmas in Cuba for nearly 30 years in 1969.

And yet the letters suggest that Castro was a man of unusual spiritual depth -- and a fervent believer in God. Addressing the father of a fallen comrade, he writes: "I will not speak of him as if he were absent, he has not been and he will never be. These are not mere words of consolation. Only those of us who feel it truly and permanently in the depths of our souls can comprehend this. Physical life is ephemeral, it passes inexorably. . . . This truth should be taught to every human being -- that the immortal values of the spirit are above physical life. What sense does life have without these values? What then is it to live? Those who understand this and generously sacrifice their physical life for the sake of good and justice -- how can they die? God is the supreme idea of goodness and justice."

Any reasonable reading of these letters would lead one to anticipate that Castro would have been an exceptional steward for his country. His laments about the cruelty of Batista's secret police suggest that he would institute a system grounded on human rights. He wrote mournfully of the slaughter of his followers after their capture: "As for the prisoners, the entrance to the Moncada Garrison could well have had the warning posted in Dante's Inferno, 'Abandon all hope.' . . . Scenes of indescribable courage were exhibited by those tortured. Two young women, our heroic comrades, Melba Hernandez and Haydee Santamaria, were detained at the Civil Hospital. . . . To the latter, still in the barracks at dusk, a sergeant . . . with bloody hands, showed her the eyes of her brother which he had just gouged out. Later that night, they also gave her the news that her fiancé, also a prisoner, had been killed."

Of his own situation he bitterly complained:

"About me, I can tell you that the only company I have is when they lay out a dead prisoner in the small funeral parlor which is across from my cell; there are occasions of mysterious hangings, strange murders of men whose health was annihilated by means of blows and tortures. But I cannot see them because there is a six foot screen blocking the only entrance to my cell, so that I may not see another human being, alive or dead. It would be too much magnanimity to permit me the company of a corpse!"

Most of all, it seemed certain that his outrage against Batista's upending of the 1952 national elections would have led him to promptly reinstate free and transparent elections in Cuba. "Any great civic-political movement ought to have sufficient force to conquer power, by either the peaceful or the revolutionary route, or it runs the risk of being robbed of it, as happened to the Orthodox [Castro's political party] just two months before the elections."

And he lamented the trend toward cults of personality in Latin American politics. "I believe fundamentally that one of the greatest obstacles to the formation of such a movement is the excess of personalities and the ambitions of groups and leaders."

But in a few short years, Castro himself would become the looming personality in the hemisphere, while maintaining an inviolate zone of personal privacy for himself. Moreover, Cuba has not had a presidential election since 1948.

Toward the end of his incarceration, Castro began a correspondence with an ardent young supporter named Maria Laborde, in which he expressed a desire for a more intimate exchange. "The inscription on your card was so beautifully written, I have set my hope on the pleasure of soon receiving a letter from you, with the only variant that you use 'tu' instead of 'usted.' Could this be too much to hope?"

Castro's wish was realized soon after his release. Although it is not widely known, he began a liaison with the devoted Laborde, who later bore him a son.

In May 1955, just 13 days before his release, a light-hearted Castro wrote to his sister, ironing out his future housekeeping arrangements: "Regarding material comforts, if it were not essential to live with a minimum of material decency, believe me I would be happy living in a tenement and sleeping on a cot with a box in which to keep my clothes. I could eat a plate of malanga or potatoes and find it as exquisite as the manna of the Israelites. . . .

"There is nothing more agreeable than having a place where one can flick on the floor as many cigarette butts as one deems convenient without the subconscious fear of a housewife, vigilant as a sentinel, setting the ashtray where the ashes are about to fall. . . . Do not think I am an eccentric or that I have become one. . . . Books alone I need."

On May 15, a triumphant Fidel, his brother Raúl, and their followers strolled past the gates of the Isle of Pines prison -- the beneficiaries of a national amnesty for political prisoners that Castro had campaigned for from his cell. Castro went directly to Havana to resume his campaign to topple the Batista government while Raul went to visit their ailing father in Biran. Angel Castro was a brawny, self-made land tycoon with whom Fidel had had a sometimes contentious relationship. Two months later, the brothers and their followers fled to Mexico. Castro would never see his father again.

Several years ago, Castro seemed to have made his peace with his father and installed a photograph of him on the wall of his office. Angel Castro died in 1956 of an intestinal hemorrhage at the age of 80 -- precisely the age at which Castro became gravely ill with a similar affliction.

Ann Louise Bardach is co-editor of "The Prison Letters of Fidel Castro," due out this week from Avalon/Nation, and the author of "Cuba Confidential" (Random House).