The Favourite: Sarah, Duchess of Marlborough
by Ophelia Field
576pp, Hodder, £20

Sarah, Duchess of Marlborough, is one of those much-biographised subjects - Byron and the Bront¿s are others - whose life has been reworked down the decades to fit any given moment. In the years following her death in 1744, Jonathan Swift put Sarah before the public as a venal schemer, a portrait that rehashed many old Tory grudges against the Marlboroughs as high-profile Whigs.

In 1839, with ideological domesticity in full swing, the otherwise scholarly Mrs AT Thomson decided that Sarah could not possibly have been an unfaithful wife, on the grounds that her household management never deteriorated, "the first signal of a woman's ruin".

By the time Thackeray got his hands on Sarah in The History of Henry Esmond, she had turned into a knockabout Judy who boxed naughty children's ears. And when Winston Churchill, Sarah's direct descendant, came to fit the Duchess into his monumental Marlborough: His Life and Times, she had changed yet again into a pushy woman whose insistence on dabbling in matters that were best left to men only served to embarrass her handsome rock of a husband.

Ophelia Field is acutely aware of these overlapping versions of Sarah and what they say, not so much about the duchess herself, as about the fears and fantasies she provoked in the people who chose to write about her. And instead of adjudicating between these competing narratives, or cherry-picking the parts that suit her own purposes, Field does that difficult thing of simply letting them lie intact within her own account of the duchess's life.

Refusing to massage away discrepancies between the Sarah of Horace Walpole's imagining (so mean that she saved on ink by refusing to dot her "i"s) and that of one mid-20th century Freudian who put her difficulties down to the menopause, Field sets before us a shifting picture of her subject. The result is not an easy read - biography has always rested on the consoling fiction that it is possible to produce a "definitive" account of a particular personality - but it is a hugely stimulating one. That it is Field's first book is something of a wonder.

At the heart of her account lies the question of Sarah's vexed position as the favourite of Queen Anne, that dull monarch who gave the lie to the idea that a woman on the throne of England automatically meant prestige and thrilling achievement. Friends since adolescence, when Sarah had been plucked from a solid, unglamorous gentry background to serve at court, the two women spent more than 30 years locked in a dance of souring love.

At first it was Sarah, with her good looks and sharp wit, who had the upper hand over her puddingy mistress. Needy and wheedling, Anne nagged constantly for more love and attention, not to mention chocolate (as Lady of the Bedchamber, Sarah had to keep the queen topped up with her favourite beverage).

In time, hurt by Sarah's careless attitude towards her, not to mention her trenchant Whiggism, the increasingly conservative Anne transferred her affections to Abigail Masham, whom she swiftly promoted to Keeper of the Privy Purse. What made the snub more pointed was the fact that Abigail was Sarah's cousin, a dumpy country girl whom she had taken under her glamorous wing.

It was in the ugly unravelling of the relationship between the queen and the duchess that its disputed nature became apparent. Sarah proceeded to blackmail her mistress, threatening to denounce her as a lesbian if she was not restored to former prominence, with all her political and financial advantages intact.

It remains a moot point whether Sarah and Anne's relationship had ever strayed into activities which, if publicly known, would have caused a scandal. But certainly there was something about both women's attitudes towards other women - Anne only too ready to become debasingly dependent, Sarah quick to accuse anyone who had disappointed her of unnatural feelings - that suggests something more complicated than simple friendship was in play. It is for that reason that one recent reworking of Sarah's life has placed her, along with Anne, in the pantheon of celebrity lesbians (although why anyone, gay or straight, would want to emulate all that vicious hair-pulling remains a mystery).

Nowhere is the subtlety of Ophelia Field's historical understanding more apparent than in her delicate reading of the relationship between Sarah and Anne. Biographers who come up against women behaving oddly towards one another in the 18th and 19th centuries usually take refuge in a formula about how female friendship was conducted in a more intense register than we are used to today. That way, they are absolved from having to engage fully with the tricky issue of how and in what ways human behaviour both changes and stays the same down the decades.

Field, by contrast, worries away at the problem, trying to feel her way back into a mindset that is both familiar and strange. As she justly says, Queen Anne probably never did anything more exciting with her favourites than chat and play the harpsichord. But the fact that Sarah was able to make capital - literally, in the sense of extracting hush money - out of the "strangeness" of the situation suggests that the queen had indeed crossed the kind of barely perceptible boundary by which every society holds itself in place.

Sarah Churchill's life was, of course, about much more than courtly cat-fighting. As wife of one of the country's greatest generals and richest men, she had access to the widest stage on which to play out her particular agenda. Ophelia Field is, once again, particularly good on the complicated relationship between the private and public spheres in the early 18th century.

Sarah was not a proto-Victorian wife, confined to the private realm, but neither was she a feminist pioneer, striding out into spaces that had traditionally belonged to men. Instead, she brilliantly exploited the pathways that lay between the two. No one understood better than Sarah how a quiet word in the right ear could result in the triumph of a political party or the fall of a great man.

At nearly 600 pages, The Favourite is too long. There is no doubt that Field has done her research thoroughly, but the evidence is too emphatically on display. Amid all the dulling detail of Sarah's comings and goings, small loves and minor hates, the cleverness of Field's overall conception of her subject gets lost. This is a shame, although nothing can hide the fact that The Favourite is an outstandingly accomplished debut.

· Kathryn Hughes is writing a biography of Mrs Beeton