No windswept moors, please Lucasta Miller analyses a monumental new biography of literature's most famous family

the brontes Juliet Barker Weidenfeld £25

Ever since Mrs Gaskell published her famous life of Charlotte in 1857, the Bronte family has inhabited a no-man's-land between reality and fantasy. As Gaskell's great, novelistic version of their story seeped into the cultural memory, it became more and more mono-dimensional, reduced to a windswept moor, three lonely Cinderellas, a misanthropic father, and a profligate son. Told and retold in print, on stage, and on screen, it has become as archetypal and romantic as Jane Eyre or Wuthering Heights, and has spawned a dubious wealth of conspiracy theories and legends.

There are no apocrypha in Juliet Barker's book, which sets out to demythologise the Brontes. As a historian, she sees it as her prime duty to sort out the facts from the fictions and to place the family within its contemporary context. Eleven years of painstaking research have produced a documentary record as full and as trustworthy as we are ever likely to get, which draws on a wider variety of source materials than previous accounts. As such, it is an extraordinary achievement.

Her description of Haworth as a busy industrial township with appalling sanitation (in one case, 24 families had to share a single privy) is a good corrective to the storm-tossed Eden of legend. Her detailed - almost too detailed - account of the Rev. Patrick Bronte's career reveals him to have been a public figure of some standing, writing to the papers, sitting on committees, and closely involved in the political and social life of the area. Her revelations about Branwell, who had an illegitimate child, are positively sensational. She clears up the mystery, which has bothered biographers for years, about his fateful affair with Mrs. Robinson, whose son he was employed to teach. New evidence shows that it really took place: it was no mere delusional fantasy, nor was it (as Daphne Du Maurier suggested) a cover-up for the worse crime of child sex abuse.

Yet for all its historical accuracy, can this biography can live up to its claim to being "the first book to strip away a century of legend and reveal the truth about the Brontes"? Biographical fact, which can be verified, is not the same as biographicaltruth, which involves a leap of the imagination and can never be other than partial, fragmentary, and subjective. The Bronte mythology does not merely consist of a catalogue of false rumours and factual errors. These, of course, can be weede d out, but the data which remains cannot exist in an interpretative vacuum. Ways of seeing - of selecting and presenting evidence - are also part of the process of myth-making, which is perhaps an inescapable condition of biography.

Barker complains that literary critics who search the Brontes' fiction for "some deeply hidden autobiographical truth'' are involved in "a subjective and almost invariably pointless task". Yet her own stance - like that of any historian - is not objective. She makes it clear where her sympathies and antipathies lie, and raises an interesting question about the role of the biographer in presenting herself as a dispenser of "justice''. The Bronte men have indeed been caricatured in the past, but one occasionally feels that Barker's attempt to redress the balance in their favour relies too heavily on cutting the women down to size. Charlotte and Emily - Anne less so - have become such icons that it is easy to understand Barker's urge to demystify them. Yet you also have to remember that without their genius the Brontes would never have become biographical subjects in the first place.

In one sense, Barker is quite right to attack the way in which the Brontes' novels have sometimes been used as biographical evidence. The man who argued, on the sole basis of Wuthering Heights, that Emily must have had a love affair with a farmhand was clearly barking up the wrong tree. You cannot prove from fiction that an actual event in the external world did or did not take place. But biography is not - or should not be - merely a record of external events. And if you accept that it has anything to do with its subjects' internal lives, it seems hard to deny that a writer's works can give you some insight, however impressionistic, into her emotional state, intellectual development, or way of looking at the world.

The question, then, for the biographer is whether he or she should be trying to represent the "truth" as it may have appeared to an unemotional outsider or as it appeared within the subject's imagination. The truth about a life - and particularly about agroup of lives - is so multifaceted, so open to interpretation, and so capable of redefinition that perhaps the biggest myth of all is that the "definitive" biography is possible.

Barker may not be able to claim to have had the last word on the subject, but her contribution will be of enormous value to future generations.