How is it possible to emerge from 25 years in prison as a rounded, tolerant and humorous human being? Ahmed Kathrada, the Indian South African freedom fighter, was in prison for only two years fewer than his close colleague Nelson Mandela. His vivid and perceptive letters, many of them smuggled out to evade censorship, help to explain the secrets of survival; for they are not so much about the details of the struggle, as about the indomitable human spirit.
Kathrada was a few years younger than Mandela, and was a less obvious hero: when I first knew him in Johannesburg in 1951, he had recently left school and seemed too gentle and self-effacing for the coming battles against apartheid. But he had an inner strength and fortitude, which came from his family, Gandhi and the communists.
Above all, like Mandela, Kathrada had a strong sense of dignity - and this is evident throughout these letters. "Is all this suffering really necessary when you can easily live a comfortable life outside?" he was asked just before he was sentenced to life imprisonment in 1964. He replied in a letter: "Of what good are these things to me when I haven't got dignity?"
The influence of Gandhi had already taught him to avoid bitterness. He saw his prosecutor, Percy Yutar, as "a sick man, a paranoiac with a burning ambition". But he resisted hatred. "Fortunately, my nature will not allow me to harbour hatred for anybody, no matter how deeply he may have wounded my feelings."
This ability to ignore provocations and insults was the key to his peace of mind in prison. "So much depends on one's mental attitude . . ." he wrote, six years after his arrest. "If one allows oneself to be excited and carried away by every triviality, life can really become miserable."
With his self-discipline and detachment, Kathrada could sometimes write about Robben Island as if it were a kind of paradise. "It's as if I'm alone on the island," he wrote one early morning in 1979, listening to the waves breaking and the wind whistling. "One feels almost driven towards the sessions of sweet, silent thoughts and the remembrance of things past."
But Kathrada was never an ascetic or a prig removed from ordinary pleasures, and some of his most moving letters are about what he was missing in the world outside. "We hear of hippies, flower children, long hair . . ." he wrote to a young friend in 1973. "I'm merely trying to establish some rapport with teenagers . . ."
He missed children more than anything, although he had none of his own, and was thrilled when they were eventually allowed to visit. "Every visit from a child brings to us a breath of fresh air . . . reassurance that we remain part of the world outside prison." He practised holding a baby after 23 years of inexperience. "I took careful note of how Prince Andrew held Fergie's little Beatrice, and I studied photos in magazines and newspapers . . . But I just couldn't hold her."
His letters describe the hunger for news of families and the world, and they record the gradual relaxation of censorship and isolation. Radio, books and newspapers were gobbled up, but some of the new fashions and tastes came as a shock.
Kathrada was at first appalled by the arrival of pop music - "a barrage of pop stuff" - but eventually some of the noise "managed to insinuate itself into my system", and he became a devotee of Nat King Cole, as well as of Miriam Makeba and Harry Belafonte. He was delighted when he could see films, particularly if they featured Peter Finch, Jane Fonda, Vanessa Redgrave or Meryl Streep. "And, believe it or not, I thoroughly enjoyed John Travolta in Saturday Night Fever. But please don't talk too loudly about the latter among serious guys . . ."
Kathrada was more interested in pop culture than the "geezers", as he called his more serious elders such as Mandela and Walter Sisulu, who disapproved of his favourite comic-strip characters, including Andy Capp and Blondie. "You see," he complained in 1988, "the 'geezers' are generally mission- educated men . . . They tend to be rather strait-laced, which immediately restricts conversations to kosher subjects, absolutely prohibits the telling of 'rude' jokes, as well as the use of four-letter expletives - even in anger!"
Kathrada provides a fascinating picture of Mandela, whom he saw at close quarters on Robben Island and at Pollsmoor - insights that I found invaluable when I was writing Mandela's biography. He could be exasperated by Mandela's self-control and reluctance to display emotion - particularly after visits from Winnie or old friends.
"We are convinced (and told him so) that if he were to be called to the office and told that he would be released tomorrow, he would return to the cell and tell us after an hour or two as if it is simply one of those everyday things."
But Kathrada was filled with admiration for Mandela's fortitude and generosity; he was always ready with advice, moral support and positive plans. He describes in detail how Mandela lovingly created gardens on Robben Island and at Pollsmoor. As Mandela became more famous and crucial to the future, Kathrada was amazed how calm and unperturbed he seemed. "Nothing in his talk or demeanour gives one the slightest hint that he is the man about whom there is such an upsurge of feeling throughout the world."
The real appeal of this book is literary rather than political. It is a charmingly written record of relationships under the most testing conditions, which could be about any group of political prisoners, anywhere. And it describes with complete modesty the triumph of the human soul. A prisoner looking through a window, as Kathrada testifies, can see the bars, or the stars. In this book, the prison bars seem to disappear as the stars shine through.
Anthony Sampson is the author of Mandela: the authorised biography, republished in paperback this month by HarperCollins, £9.99