Last week it was announced that Nelson Mandela's 13-year-old great-granddaughter, Zenani, named after one of Mandela's two children by Winnie, was killed in a car crash on the way home from the World Cup opening party. The driver was allegedly drunk. Mandela said that he would not be attending the official ceremony. That a great-granddaughter of the most famous man on earth could have been entrusted to a drunk beggars belief. I raise this tragedy because it seems to me to be horribly familiar. Mandela's families  there are two  have suffered appallingly as a result of his fame and his devotion to the anti-apartheid struggle. The succeeding generations have been racked by instability, and they continue a sometimes bitter competition for influence, money and  as some of them see it  ownership of the Mandela brand, competition that boiled over most recently when the ageing Mandela was enlisted to support Zuma's election.

No one has been affected more than Winnie Mandela. Married young and soon separated from her much older husband, she was brutalised by the regime to such an extent that  in the charitable version of her life  she was so disturbed as to be complicit in terrible violence towards a young boy, Stompie Seipei. She was almost certainly guilty of ordering the killing by her "football club" of her own doctor and devoted supporter, Abu-Baker Asvat, when he was subpoenaed to give evidence after he examined Stompie shortly before his death. The doctor's receptionist was Albertina Sisulu, whose husband, Walter, was Mandela's first sponsor and closest ally. Although Sisulu described the events in detail at the time, she claimed in 1997 at the Truth and Reconciliation Commission to have seen nothing. The only conclusion a reasonable person could come to is that she was leant upon to protect the Mandela name.

One of Mandela's sons by his first marriage, Thembi, died in a car crash in 1969, and another, Makgatho, died of Aids in 2005. Thembi, although living in Cape Town, had never visited his father on Robben Island. Both sons were left rudderless by Mandela's desertion of Evelyn, their mother. And Winnie has often said that her brief life with Mandela was appallingly insecure and that she hardly had a family life: he had famously proclaimed at his trial in 1961 that the struggle was his life.

David James Smith finds evidence that Mandela was in many ways a traditional African, who regarded himself as head of the family and expected to be obeyed. This is hardly a surprise. He also produces evidence that Mandela may sometimes have hit his first wife, Evelyn, and that he had a child by another woman. Evelyn retired eventually to the Transkei and lived on in embittered silence, although she talked to an earlier Mandela biographer, Fatima Meer, saying that Nelson was the only man she had ever loved and that he was a wonderful father.

It has never been a secret in Johannesburg that there was a different side to Mandela, that of a compulsive womaniser with a taste for fame, a dandy, a boxer, a man with an iron will. I was told some years ago by Breyten Breytenbach, the Afrikaner poet, who was imprisoned for seven years after a tip-off from the African National Congress (ANC), which distrusted his secret mission to South Africa, that the ANC leadership were utterly ruthless; they had been exposed to hardships and humiliation in their own country that made them, as he put it, quite different. As this book makes clear, the white, mainly Jewish, communists who were in alliance with the ANC lived their lives in the affluent suburbs with swimming pools and servants. Even when Mandela was living in hiding at Liliesleaf Farm, to the north of Johannesburg, the nominal owner of the farm, a communist called Arthur Goldreich, was taking riding lessons, while Mandela lived in a bare servant's room to maintain his cover as a "garden-boy".

Despite having innumerable reasons to hate white people, Mandela never allowed himself or the ANC to take the strictly Africanist route followed by the PAC (Pan Africanist Congress of Azania), and now increasingly by the ANC youth league. He believed that there could be no compromise with the principle of non-racialism. Towards the end of the 50s he was recognised by his comrades as the man to symbolise the ANC. It was he who brought the ANC round to the idea of the armed struggle and he was the first leader of the military wing, MK, the spear of the nation. It was understood that Mandela would sooner or later be captured and possibly hanged. That was his role, and he accepted it, while understanding the consequences. Yet Mandela's letters from prison demonstrate that he never stopped feeling guilty about leaving Winnie and his daughters at the mercy of the regime, though he seems to have had less intense feelings for his first family. One of his granddaughters has said that no one who went into the struggle should have had a family.

The greatness of Mandela is not diminished by his adulteries and his desertions or his inability to demonstrate affection for his children. It lies in his unshakable resolve to produce a fair and humane society for all in South Africa, a legacy now under threat. It seems to be true, however, that he found it far easier to feel deeply for the oppressed than for his own family. Smith tells a moving story of his daughter Maki's placing his hand on his dying son Makgatho's hand as a gesture of reconciliation with his first family, but Mandela withdrew immediately. "He was frozen," said a granddaughter who was present.

This book is valuable and fascinating, in the new detail it brings to the account of Mandela's life, from his first acquaintance with the ANC to his imprisonment in June 1964, an imprisonment that caught the imagination of the world but destroyed his family.

Justin Cartwright's most recent novel is Too Heaven By Water (Bloomsbury)

There could not be a more poignant moment for the release of a book about Nelson Mandela's personal life, and the complex interplay of political imperatives and family commitments that have bedevilled it. On the eve of the World Cup that he was to preside over as his final glorious public act, Mandela's great-granddaughter was killed in a car accident; the driver, a member of the extended Mandela family (although not related by blood), has been accused of being drunk. Young Mandela is the backstory.

Mandela's older son, Thembi, died in a car-crash while his father was in jail, in 1969; so alienated was the young man from his father that he had not visited him in prison on Robben Island. Mandela's younger son, Makgatho, was an alcoholic who died of an Aids-related illness in 2005. According to David James Smith's informants, he was a gentle sort deformed by his authoritarian father's incapacity for affection and "unrelenting scrutiny".

Mandela's granddaughter Ndileka tells Smith Makgatho descended into alcoholism because of these deep wounds, and recounts a troubling story about how she failed to affect a death-bed reconciliation: Mandela "was frozen. He just could not accept his own feelings. Granddad can be affectionate with strangers but he is completely cut off from his own family."

Thembi and Makgatho's mother was Mandela's first wife, Evelyn, who died in 2004. She left him, she claimed in the divorce papers, because of his womanising, neglect and violence; immediately the divorce came through, he married Winnie, and there has been tension between the "first family" and the "second family" ever since. The womanising allegations have been aired before; now, Smith names names: the singer Dolly Rathebe, the ANC women's leader Lilian Ngoyi, his legal secretary Ruth Mompati, who allegedly bore his son.

The violence allegations are the most serious: Evelyn claimed Mandela beat and throttled her, and threatened to kill her with an axe. Smith spends some time trying to understand how Mandela could have done this: he was "very patriarchal", and perhaps, given all the political pressure he was under, he simply "blew a gasket" in what was obviously a bad match. He comes to the conclusion that there must be "at least some credence" to the allegations, despite the fact that Mandela has categorically denied them, that they were not tested in court, and that they might have been fabricated or exaggerated by the aggrieved complainant. This is strong stuff, and is part of Smith's stated intention, from the outset, "to rescue the sainted Madiba from the dry pages of history, to strip away the myth and create a fresh portrait of a rounded human being".

At the very least, this is a long-overdue exploration of the making of the Mandela myth; one that refreshes a somewhat stale and overcrowded field. Smith sets the territory by looking at the stark difference between Mandela's account of his father, a Thembu noble and a colonially appointed headman, and the documentary evidence provided by the colonial archive. He then effectively demonstrates how Mandela's memoir was designed to "boost" the cult around him: although Mandela instructed his comrades to insert the line, "I led a thoroughly immoral life", into Long Walk to Freedom, an "admission of immorality might have detracted, or at the very least distracted, from his heroic reputation". And so "history had been revised".

Of course, this last comment is the very definition of memoir, all the more so for someone who has exercised such tight control over his public image. Mandela has made a political fetish of his biography: as he was in chains, so too were all South Africans; as he liberated himself and forgave his oppressors, so too can we all expunge the hate from our hearts. For this reason, the most striking and valuable parts of Young Mandela are the rare occasions where we hear Mandela's unmediated voice, in a series of exquisite letters to Winnie and his daughters from jail. Here, away from the public eye, he articulates acute emotional intelligence and deep regret as he recounts the way his calling has denied his children a normal family life.

The book also includes some well-researched recapitulations of key political moments in Mandela's early life: the retelling of his time underground stands out, as does the description of the "double-life" of his white comrades. But I put the book down not so much with a clearer understanding of the making of Mandela as with the kind of headful of gossip you carry away after spending too much time in a small town.

Perhaps this is a comment on the small town of the Mandela industry itself: Cranford-on-the-Highveld. Like all gossip, some of it is illuminating, but much is gratuitous, unsubstantiated and even malicious: Smith is obsessed with the sexual goings-on of the white left, which tell us nothing about Mandela's own infidelities; he uses unnamed sources to have a go at Maki, Mandela's oldest daughter, for declining to be interviewed; he also twice reports on "suspicions" about the bona fides of Mandela's co-accused Govan Mbeki, with no evidence to back it up. More seriously, he has no firm corroboration of the allegation that Mompati bore Mandela's son, something she firmly denies.

Smith also does not give enough weight to the way revisionism and self-mythologisation is often a balm to the wounds made by history rather than an act of willful intent. Often, too, he does not look closely enough at the reasons for the disjuncture between Mandela's public memory and the conflicting evidence he has found; this is most evident in the case of the colonial record about Mandela's father.

Ultimately, despite his strong research and laudable intentions, Smith falls into the mythbuster's trap. Some people "won't hear a word against" Mandela, he writes, and so sets himself the task of finding all the "words against" he can. In so doing, he sometimes loses sight of the primary reason for biography, which is to make sense of a life within its times, and to bring us closer to understanding its subject.

Mark Gevisser is the author of A Legacy of Liberation: Thabo Mbeki and the Future of the South African Dream.