In 1919 Vera Brittain was shattered to discover that her slightly younger fellow-students at Somerville College, Oxford found her memories of the War (she always gave it a capital letter) boring. She had lost her brother and his three best friends, one of whom she was in love with, had served with devotion in the VAD (Voluntary Aid Detachment - women who volunteered to nurse the war-wounded under the aegis of the Red Cross), and, at the war's end, was near a nervous breakdown from grief and overwork. When she spoke of these experiences at a meeting of the college debating society the audience responded unkindly. That evening, "too miserable to light a fire or even to get into bed, I lay on the cold floor and wept with childish abandon. 'Why couldn't I have died in the War with the others?' I lamented ... 'Why couldn't a torpedo have finished me off ... I'm nothing but a piece of wartime wreckage, living on ingloriously in a world that doesn't want me!'"

I must confess that, reading this book soon after its publication in 1933, I was on the side of the young women who had caused this misery. What came across to me then was her self-importance and heavy earnestness, and I (a very new-fledged pacifist) disliked the way she wrote as though she alone had suffered in the war, romanticising it even as she abhorred it. Every adult I knew had been through it, and none of them saw themselves as wreckage living on ingloriously and unwanted. Of course war was unspeakably vile - but what a tiresomely melodramatic woman this one was!

So it was a surprise to pick her book up now and discover how very good it is.

Brittain could indeed be self-important, but more to the point are her lively intelligence and genuine passion. She was brave, and her strong feelings would always express themselves in action. And she was honest. Why do today's reviewers congratulate autobiographers such as I for trying to be honest as though it had never been done before, when there she was in 1933, as blazingly honest as anyone can be? We would know nothing of her weaknesses and occasional absurdities if she had not recorded them so scrupulously, and how much more we understand about the nature of that hideous war, after she has evoked the innocent idealism with which her brother, her fiancé Roland Leighton and their friends went into it - and how they ended.

Her rage at the limitations of pre-war provincial society is also an eye-opener, and her successful struggle to escape makes a good story in itself, as well as explaining why those beloved young men played such a dominant part in her emotional life. No less admirable is her vivid account of her initiation as a nurse, the dreadful conditions in which she worked, and the way she was supported by believing she was sharing at least a little of what the man she adored was enduring. And the appalling moment when she ran joyfully to take the phone call which would tell her he had arrived home on leave, and was told he was dead.

Surely I must always have been moved by that? Yes, the shudder of recognition that came over me now when she was buying the dress she was going to wear for him, and I remembered what was going to happen on the next page, proves that I was. She did sometimes slip into over-writing, but she could most powerfully convey the indescribable when she had to, and at least I recognised that. Otherwise, however, my young self was doing something from which I recoiled when I met it not long ago in a silly woman who told me she detested the stories of that marvellous writer Alice Munro, "because she writes about such dreary, low-class people". I had failed to understand Vera Brittain because she wrote about people whose extreme sexual innocence (she and Leighton hardly ever even held hands) appeared to me ridiculous, and whose eager embracing of the war as "glorious" shocked me. It is shaming to see that I reacted like an illiterate nincompoop.

Now I can see clearly how remarkable Brittain was. Her stuffily middle-class family could not have been more conventional. Her parents never questioned that their son should go to a university if so inclined, but saw their pretty little daughter's wish to do the same as absurd; nor was it thinkable that she should go anywhere, even to shop in nearby Manchester, unaccompanied. As for being alone with a man - that was so indecent that when, on just one daring occasion, she and Leighton managed to engineer a short train journey together, they were too shy to talk to each other naturally until the last moment.

But in spite of all these inhibitions Brittain went on driving her parents mad with Oxford, Oxford, Oxford; why couldn't she go to Oxford? Eventually she won. "What would people think?" always weighed heavily with them, so the balance was tipped when a visiting university extension lecturer, who struck them as distinguished, took her desire as natural and within her reach. But she herself had brought it to the tipping point; and with no coaching or encouragement got herself through the entrance exams with record speed, teaching herself Greek for the purpose. Observing how her brother and his friends read and talked and used their minds, she resolved to achieve acceptance as their equal, and she made it.

She was always to make any "it" she aimed for, although she often described herself as nervous and unsure, partly no doubt for fear that acknowledging her own abilities would look conceited. It is not possible to be quite sure how well aware she was of her own cleverness and strength of will. But she was highly strung and emotional, so probably she really did experience much of the anguished uncertainty she describes before each triumph; and once she was committed to Roland Leighton, triumphs became less important. When she witnessed the elation with which he and the other boys welcomed the prospect of fighting and perhaps dying "gloriously" for their country, her own imagination caught fire. She had enough of it to envisage horrors, but she felt she too ought to experience them in order to stay with the boys - particularly with Leighton - in spirit. As she would say when looking back, "The War made masochists of us all".

So she withdrew from Somerville and began nursing the wounded, and the grimmer her work, the more dedicated to it this hitherto sheltered young woman, who had never put her hand to any physically disagreeable task, became. One can only feel awestruck at the courage and endurance that she and her fellow VADs discovered in themselves.

Then came the deaths: Leighton's, her brother's and those of their two closest friends. They turned Brittain into a committed pacifist. There had always been sane instincts in her even when she was at her most romantic: she had never, for example, demonised German soldiers whose deaths she knew were as pitiful as those of Englishmen, and her objection to society's attitude to women was always steady and sensible. Now her future path was determined, although it was some time before she could see it.

Her recovery from the ravages of grief and four years of physical exhaustion was long and painful, not helped by her plunge back into hard work at Oxford almost at once. But it was helped by a relationship she formed there: her enduring friendship with the novelist-to-be Winifred Holtby. Both young women had brought their parents to heel by then, so after Oxford Brittain's father helped her financially to start life in London, where she shared a studio flat with Holtby, and they both scraped enough money to live on by part-time teaching and lecturing.

These activities bought time for what really mattered: turning themselves into writers. Holtby was a novelist by nature. Brittain forced herself to write two novels which got published, but was more at home as a journalist - a columnist rather than a reporter - and was soon writing and lecturing on behalf of the League of Nations, which seemed at the time a useful way of working towards world peace. And, as her daughter Shirley Williams says in her introduction to the new edition of Testament of Youth, she would continue all her life "writing, campaigning, organising against war".

Although still in her 20s, she was convinced that she would never marry. She didn't want to - loving meant suffering, so out with it! She saw this as a tragic decision and therefore, given as she was to extremes, she embraced it with a martyr's enthusiasm. So there is inevitably something slightly comic in the fact that, once she had established that he revered work for the equality of women and the economic security of the working man, and that he also granted that her work was more important than any domestic involvement, she capitulated (though grudgingly) to the wooing of George Catlin, and married him. Cross though she might have been at the time if forced to admit it, this does make a satisfyingly happy ending to her wonderful book.