The UK Independent reports that 21 letters from prison to be published in English for the first time reveal the spiritual side of Castro who has been in failing health for the past 8 months.
Writing to the father of a dead comrade, Castro said: "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? ... God is the supreme idea of goodness and justice."
However, other letters are also boastful, the Independent says, while some display his fury.
Though published in Spanish in April 1959, just months before he and his revolutionary comrades overthrew the dictator Fulgencio Batista and seized power, the letters have never before appeared in English. This week a new collection containing the letters is to be published in the US - Mr. Castro's most fervent enemy over the years.
A report in a US newspaper reveals that the letters begin several months after the 26 July 1953 attack on the Moncada barracks, which proved disastrous for Mr. Castro and his men, at least 60 of whom were killed.
In the aftermath of the attack, Mr. Castro was sent to prison. While he was a prisoner his wife, Mirta, accepted some money from her brother, Rafael Diaz-Balart, the country's deputy interior minister.
Mr. Castro wrote a furious barrage to his wife. He wrote: "I never imagined that Rafael could be such a scoundrel and that he has become so corrupted. I cannot conceive how he could have so pitilessly sacrificed the honor of and name of his sister, exposing her to eternal shame and humiliation."
Writing in The Washington Post, the co-editor of the collection, Ann Louise Bardach, says the letters "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."
Ms Bardach suggests that anyone reading the letters would be convinced that Mr. Castro was a committed democrat - determined to hold free and fair elections. In one letter he writes that "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".
Photo: Fidel Castro and Mirta Diaz Balart with their son.