WALKING IN THE SHADE: Vol II of 'My Autobiography', 1949-1962 by Doris Lessing, HarperCollins pounds 20
What do you when the hopes, beliefs and ambitions which sustained you in your youth seem, in retrospect, a kind of lunacy? You can deplore your young self. You can mock and patronise. You can seek to justify. A more difficult course, neither self-hating nor self-indulgent, is simply to explain - to face up to what you were by reconstructing the mind-set of a whole era. This is the path taken by Doris Lessing in the fascinating second volume of her autobiography.

Under My Skin, the previous volume, told of her childhood in Southern Rhodesia, her two marriages and her early writing career. This one picks up the story in 1949, as she arrives in Britain at the age of 30 with pounds 150, the manuscript of her first novel and her young son. The money pays the rent on a flat in London W11, the novel is accepted for publication, and the wakeful, demanding child soon settles down. In dingy, smoggy, bomb-damaged post-war Britain, Lessing starts to feel at home. But the real story isn't this but something else: the life and loves of a woman passionately committed to the Left.

Doris Lessing joined the Communist Party in the early 1950s. Even at the time, it seemed to her a strange and "neurotic" thing to do. She was suspicious of the Soviet Union and didn't like the atmosphere of King Street (the British CP HQ). She hated meetings, was not a natural joiner, believed the stories of Stalin's atrocities, felt sick when comrades casually argued that you can't make omelettes without breaking eggs. On the other hand, her allegiances had long been socialist, and were reinforced by the class divisions (shocking to a colonial) which she saw in Britain. All the generous, colourful, literate people she met in London were communists. And she wanted her politics to be more than skittish, more than a way of annoying mummy or shocking the bourgeoisie. So she joined.

Joining didn't just mean signing petitions and having a Party card. There were also foreign congresses and writers' delegations. Lessing describes in detail a trip to the Soviet Union with, among others, Naomi Mitchison. While Lessing seethed at the "noisy insincerity" of what she heard and saw, Mitchison was less inhibited, complaining that the Soviets were no longer tolerant of free love (she had once bathed nude in the Moskva with her lover, she tells them) and satirising socialist-realist paintings (to her suggestion that a cow in one of these paintings looks as if it needs milking, the Party official replied severely "Soviet cows are well treated").

Lessing recalls such scenes with amused but also appalled fascination, wondering how it is that she and thousands of other western intellectuals - "useful idiots", as Lenin called them - could have supported the most brutal tyranny of our time. "How ridiculous it does seem now - that we took ourselves so seriously," she says, and wonders if sentimentality, mass hysteria or a "delayed or displaced collective adolescence" were to blame. But this is surely too harsh, on herself and on others. A truer answer lies in a letter she write to E P Thompson in 1957, when she speaks of being a communist not as "a question of intellectual standpoint, but rather a sort of sharing of moral fervour".

Suez, Aldermaston, the Soviet invasion of Hungary ... Lessing worries that all this ancient political history will bore her younger readers. She needn't: those moral fervours are recognisable and bring it to life. Besides, as she says, politics seeped into everything then. Her memories of Britain in the Fifties - damp houses, draughty windows, utility furniture, one- bar electric fires, awful food, undrinkable coffee, parish-pump mentality, everyone in bed by ten - evoke the special texture of the Cold War.

Politics also seeped into love, the other great illusion - or disillusion - in her life. It's not that she lacked lovers. First came Jack, who was Czech and a psychiatrist at the Maudsley; later Clancy Sigal, an American journalist and Trotskyist romantic; others are briefly mentioned but these were the two that count. Both had deep relationships with Doris Lessing, but neither was ever more than passing through. They made no secret of this - there were always other women in the background - and, since they never promised anything, couldn't be accused of letting her down. All the same, as she says, "there is no fool like a woman in need of a man", and just because she doesn't whinge about these men doesn't mean she came out unscathed. Their boyish energy may have been charming, but some of their attitudes seem, now, neanderthal.

Doris Lessing makes an analogy between love and politics: they are both about entertaining hope, and long after that hope has died or become ridiculous the poor addict will try to keep it alive. She, too, suffered from excesses of fond expectation. There were also the emotional demands of her mother, who followed her to London, not letting her get away. It helped that she had a therapist to go to, Mrs Sussman, at a time when having a therapist wasn't the norm. But her saving grace was that (a phrase she keeps using) she didn't put all her eggs in one basket. One of the other baskets she put her eggs in was called fiction, and this helped her to stave off madness. "All around me, people's hearts were breaking, they were having breakdowns, they were suffering religious conversions," but she, caught up in yet detached from this, was able to write The Golden Notebook.

She doesn't have much to say about her life as a writer, since the writer's life, she says, is mostly dull: that daily routine of procrastination and wool-gathering, the low-level depression that feeds creativity. But she does write interestingly - and corrosively - about current publishing practice and the commodification of authors. She also writes well about her forays into theatre, at the Royal Court and elsewhere, and laments their failure. A number of her books she considers failures, too, including The Golden Notebook, but at least with books she has only herself to blame. She is pleased when readers, including men, write her appreciative letters, but when they say a novel of hers has changed their life she finds that harder to believe. The tone is relentlessly disabused.

Some of the angriest passages here are reserved for arrogant or insensitive "experts" who have made her life more unpleasant than it might have been - whether property experts or doctors or political activists. They happen to have been men, but it's doubtful if she is making a feminist point here. She finds the sisterly vogue for denigrating men (as wimpish, impotent, inconsequential, etc) no less boorish than old-style misogyny. And she thinks Englishmen, despite our cold-fish reputation, "the most romantic men in the world".

As this suggests, Lessing likes to combat current pieties: always has, always will. The risk, she knows, is that the reader will think "here is just another old bat with bees in her bonnet", and her breezy opinions do sometimes make you wince. The preachiness of her book will offend those who think autobiography a matter of anecdote. She does, in fact, tell a good many anecdotes, but they always serve some larger point about the Zeitgeist. Famous names appear, without being dropped. Henry Kissinger, John Osborne, Joshua Nkomo, Kenneth Tynan, Joan Littlewood and Bertrand Russell are here because she met them - but so, at greater length, are the landladies she rented rooms from and the builders who did up her north London house. Few celebrated writers are as indifferent to celebrity.

She sounds deeply unimpressionable, as if, having been easy in the past, she's determined to be difficult - or that, once credulous, nothing will get past her now. Walking in the Shade is, among other things, an anthology of insights and apercus. Her understanding of the ways of the world has led to what she calls "attempts to turn me into a wise old woman". She disapproves of these: she doesn't want to be venerated, to be the figurehead for yet another dangerous faith. If she has anything to teach, it's that working for political change is a slow, patient, unglamorous business - and that inner spiritual development is important, too. Her own path has been Sufism, but she is reluctant to go into that here.

Her last volume was criticised for its emotional reticence, especially on the matter of walking away from two small children. She feels indignant about that charge - "it seemed to me obvious that I was bound to be unhappy and any intelligent reader would understand that without ritual beatings of the breast". In any case, she can hardly be accused of reticence here. Towards the end come 15 pages of sexual candour. They aren't "confessional" in a conventional way - it's her mind which Lessing wears on her sleeve, not her heart - but they're hard-won from a lifetime's experience and, in places, very funny.

Doris Lessing is not a writer, like (say) Virginia Woolf, who leaves the language different from how she found it. But she has helped change the way we think about the world. Walking in the Shade records another crucial stage of her education, and ours.