DAPHNE DU MAURIER, by Margaret Forster (Chatto and Windus, $306). WHEN, in her 70s, it was suggested to Daphne du Maurier that she might write her memoirs, she bridled vigorously at the idea: ''It will be the yawn of all time,'' she grumbled. She had just got over being made Dame Daphne du Maurier, which she thought made her ''sound like something out of a pantomine''. This self-deprecation was characteristic of du Maurier. She hated fuss and show, and all the trappings of literary greatness that fame had constantly tried to thrust upon her. What would she have made of this? There was a distinct shock-horror-probe element to the launch and serialisation of Margaret Forster's biography. Billed as its star attraction was a salty revelation about the writer's ''highly significant friendship'' - read ''lesbian intrigue'' - withGertrude Lawrence. Yet this book is no sop to sensationalism. It is a sensitive, level-headed and incisively analytical evaluation of not only du Maurier's fascinating character but the mechanics of her creativity. Du Maurier was a phenomenon: a bestselling novelist whose work stands on the peripheries of great literature. If Rebecca elevated her into the sphere of the serious writer, the popular escapism of her historical romances earned her censure for frivolity. The more books she sold, the more she enraged the critics: her obvious talent prevented them from dismissing her into a facile category of sub-literature. A fine line divides du Maurier's life from her work. ''The people I write about in books are more real to me than the people I meet,'' she once wrote to her young daughter. She barely tolerated reality in fiction and used it as a springboard for fantasy - to provide a credible basis necessary to the suspension of disbelief. Perhaps du Maurier was never a great writer because she lacked sufficient interest in the human condition. Yet what makes Rebecca such a powerful and enduring novel is the perception of Manderley as a human presence in an almost tangible spirit of place where fantasy holds sway over reality. The real world was merely a second home to du Maurier: she was certainly an eccentric. Menabilly, the model for Manderley in Fowey in Cornwall, was literally her spiritual one, and as her source of inspiration it came before all else (''Houses are not like marriages; one cannot just walk out and leave them''). She poured vast amounts of money into it, even though it was a severely wasting asset - infested with rats, bats and beetles - and could never physically be hers, but destined to pass to someone else under the terms of a trust. When her lease finally came to an end she suffered something very close to bereavement, missing the house ''as one misses anyone who has died and whom one loved''. Du Maurier was rarely at ease with anyone but herself, and even then she felt she had to put on an act. To be able to write, she carried an almost pathological need for solitude to the point of misanthropy; frequently to the point of jeopardising her marriage. Her young writing career had been overshadowed by the need to be an emotional stay to her suffocatingly doting father, Gerald, who, despite his brilliant career as actor-manager, became permanently depressed and overtaken by a sense of his own futility. She was hoping for a contrast to Gerald in the man she married, the Grenadier who was later to become General Sir ''Boy'' Browning. Since her affair with the actor Carol Reed, she had had a horror of being trapped into a relationship. ''It will take me at least five brandies, sloe-gins and a handkerchief of ether to push me to the altar rail,'' she anticipated. Although they and their three children were, at times, a very happy family, they were not ultimately a compatible couple. He was never in tune with her mercurial character, she with her hate of things formal and official, was never General's wife material, often feeling made to feel a failure ''because I can't organise tea for 800''. She was nevertheless grief-stricken when he died. Her sense of humour, with its nose for pretence and absurdity, remained intact, however. Reading condolence letters she recalled one which expressed its deep regret: ''I once met your husband 40 years ago in the post office.'' What of du Maurier's sexual perplexities? Forster claims she suppressed a latent lesbianism which surfaced first as an advanced form of finishing-school infatuation, and then at landmark stages in her life during particularly unstable moments in her rocky marriage. Du Maurier protested her abhorrence of what she called ''Venetian tendencies'' in other women far too vehemently for her condemnation to be taken at face value. She gave official recognition only to the less contentious aspect of her personality, her ''boy-in-the box'': her yearning, since childhood for boyishness and the freedom it afforded as opposed to its sexual possibilities. This was more insidious than it sounds: her father had always let her know he had wanted her to be a boy (and given the teenage Daphne such character-forming encouragements as the wish that when he died, he could come back as her son). Daphne wanted to produce the boy she could never be and in middle age felt she had punished her daughters for not being sons. Forster alleges two relationships: one, with Ellen Doubleday (wife of the American publisher), which du Maurier pushed as far as she could up to the perimeter fence of kindred-spirit amity without gaining the land on the other side; the other, a full-blown affair with Lawrence. Forster's provocative thesis is that du Maurier's sexual and intellectual identities were so closely bound up in one another that without their dual influence ''there would have been nothing [no creative output].'' This is the only aspect of the book which appears unconvincing: the sub-plots of her life strained and served up as a rationalisation for her work are hard to swallow. Forster, as she has done elsewhere, observes penetratingly the tyranny of the trivial over du Maurier's old age. Towards the end of her life, du Maurier became an increasingly reclusive Beatrix Potter-type figure, obsessed with the ruination caused by tourists to her beloved Fowey. She had long stopped writing, having tried unsuccessfully to break away from her type-cast as romantic best-seller into more experimental territory. Forster writes eloquently, with a strong empathy for her subject, which captures du Maurier's personality, bringing her back to life with all her fascinating contradictions, her brilliance, her sense of humour and her high spirits. This is a wonderful book. Buy it.