It is not ideal holiday reading, for two reasons. First, it will make you anti-social; this book demands to be read in a single session. Second, it will make you angry. Harriet Sergeant's Among The Hoods will cast a shadow over the sunniest day. This is a chronicle of waste, loss and tragedy.

Harriet Sergeant is a brave girl. She wanted to know what is going wrong in South London, to turn so many young men (mainly but not exclusively black) into criminals and gangsters. So she walked the mean streets to find out, befriending some pretty fearsome characters along the way. She is not only courageous. She possesses considerable reserves of moral depth and human sympathy. These youths had been hardened in a culture of violence. In their presence, most of us would only see the bad, frightening side. To the Dashing White Sergeant, these were lost boys who needed help and love.

Our author is tough-minded. She does not sentimentalise wrong-doing. She agrees that society needs to be defended from the Hoodies and their ilk. But the best – and cheapest – way to protect the rest of us would be to prevent them going wrong in the first place. Harriet identifies two root evils: the absence of fathers, and illiteracy. To help them to become men, boys need male guidance. Harriet's youngsters do try to make sense of the world around them, but in the absence of hard-working, law-abiding role models, they are unable to do so in any useful way. They want money; indeed, they dream of riches. But they can only see two routes to wealth: drug-dealing and robbery.

Miss Sergeant tries to interest them in finding work. These kids are not thick. She is convinced that one of them would make a good salesman. But there is a basic problem. They are all functionally illiterate and innumerate. This also undermines their already fragile self-esteem, and drives them back to the comradeship of gang life. At one stage, Harriet persuades a businessman friend of hers to have an informal interview with the potential salesman. She has briefed her friend about the criminal record and the ghetto mannerisms; he would still like to help, and appease his social conscience. It does not work. The encounter is like a wary meeting between members of two different species.

In all her dealings with the youngsters, Harriet Sergeant shows a patience which is all the more admirable because one doubts if it comes easily to her. But she does describe some encounters which clearly drove her to the extremes of exasperation, all the more infuriating because she was unable to express it. In an arbitrary and mostly pointless fashion, officialdom intersects with these boys' lives, and Harriet frequently found herself negotiating, even supplicating, on their behalf. She describes a bureaucracy which is usually incompetent, uninterested, demoralised and ill-led. Those she met were often suspicious of her: neither she nor they use the words, but there is a continuous sense of "What's this trouble-making upper-middle class bitch doing poking her nose in round here?" Even so, she decided that complaining would do more harm than good. One single official receives a favourable mention, because he was honest and enthusiastic. None of the rest could be bothered. There was no question of displaying initiative, of going the extra mile, of responding to the urgent need to turn round these boys' lives. The pen-pushers just followed routine and the rule-book.

That could have disastrous consequences. The failed salesman had an evil cradling: five years of neglect and worse. He was then placed with a loving foster mother. Everything improved. Then, when he was 11, she reached the obligatory retiring age for foster mothers. She wanted him to stay: he wanted to stay, but rules are rules. Deaf to all entreaty, the social workers took him into "care": that Orwellian misnomer. His foster mother's good deeds were all undone.

The bureaucracy creates another problem. It operates on the basis of form-filling. Yet these boys are incapable of filling in a form and too proud to admit it. One might have thought that over the years, the official world would have identified that difficulty and found a tactful way of addressing it. But that would require common sense and common humanity. They are not in the rule book.

The boys' illiteracy had sabotaged their secondary education. Not unreasonably, secondary schools expect their pupils to have mastered basic skills. Again, the lost boys were too ashamed to face up to their weakness. Instead, they started playing up, until they were excluded from school, on to the streets, where the devil found work for idle hands.

No-one should blame the schools which did the excluding. Such troublesome boys can destroy all the other pupils' chance to learn. But exclusion should not just mean turning a boy loose. A few years ago, the Tories were proposing sin-bin schools. Excluded pupils would be required to attend them, until they had worked, behaved and repented their way back to the normal school. It sounded like a good idea, but seems to have been dropped. That is a pity.

There is a further problem. These boys need legitimate means of expressing their malehood, through sport and adventure. Otherwise, they will find their own way to adventure, via mugging or burglary, or by carrying a knife, which could end up in somebody's chest.

Something must be done. We cannot continue to acquiesce as young boys are turned into feral humans: their own lives wrecked while they take revenge by blighting the lives of others. For a start, there must be zero tolerance of illiteracy, innumeracy and truancy. We have to make schools work. Second, the inner-city young must have opportunities for healthy exercise. Third, we need to examine the way that social workers operate; there ought to be zero tolerance of the idleness and slop which Harriet Sergeant describes. That is only a beginning, but it is all practicable.

For some years now, Harriet Sergeant has specialised in social exploration, forcing the urban jungles of South London on to the agenda, compelling polite society to face up to challenges which it would rather ignore. This is work of the highest value, and everyone should read her latest book. The ending is bleak. Harriet's best efforts have earned in a miserable harvest. We may never be able to repair the damaged youngsters whom she befriended, because it is too late. That makes it all the more urgent to prevent other children at risk from sliding into ferity.