Kate: The Woman Who Was Katharine Hepburn, by William Mann, 601pp, Faber, £18.99

England's Mistress: The Infamous Life of Emma Hamilton
by Kate Williams, 414pp, Hutchinson, £20

Here's a choice of performances. Kate Williams's heterosexual Emma Hamilton, posing on a dais in her mini-dramas called "Attitudes", desperate for attention. Or William Mann's bisexual Katharine Hepburn, being attitudinous on the movie screen, rather hoping for attention. The woman performer as victim, passed from whorehouse to controlling men, learning to use her desirability and notoriety first as an artist's model, then by inventing the Attitudes. Or the woman performer as non-victim, independently mobile, learning to use her asexual singularity as a star, then as an actress, then by reinventing herself as an institution.

Central to both was a genuine love for a battered, married hero - Horatio Nelson, Spencer Tracy - who was complicit with his nurse: Emma cut up Horatio's meat and forked it into him; Kate stocked Spencer's quarters with chocolates to wean him off booze. Of course, the Hamilton-Nelson affair was conventional, producing two bastard daughters (one died), while, in Mann's main quasi-revelation, the Hepburn-Tracy relationship, though possibly physical at the start, was a passionate friendship - Tracy was also bisexual, his guilt over homosexual episodes being the source of his demons and drinking.

Nelson at his death left Hamilton as a "bequest to the nation", and she saw herself as the People's Trollop ("I will ... beg through the streets of London and every barrow-woman shall say, 'Nelson bequeathed her, to us'"). But the government declined the inheritance; she bankrupted herself of what marginal income men provided for her in preserving Nelson's overpriced love-nest as their monument, was imprisoned for debt and died of liver failure in Calais. Tracy's death before the release of the couple's final picture, Guess Who's Coming To Dinner, established Hepburn as a national treasure to her end at 96, an elevation reinforced by Garson Kanin's fictional script about, rather than for, the couple - his 1971 "intimate memoir" "Spence and Kate". After Tracy's death, Hepburn declined to buy the Californian bungalow they shared, since her permanent home was a New York house; harsher than any of Mann's reproofs about her grande damedom is his note that her father managed her money until she was 54, after which her longest-serving woman friend took over. Now that is shocking.

Williams wants the reader to grow in affection for her subject and, despite being irritated by her overuse of the Heat-style idiom of celebrity, I did come to care more about Hamilton. Vulgar, yes; silly, certainly; compliant towards men - but that was the job description of being several libertines' passed-on mistress, then hostess and wife to Sir William Hamilton. She was also frank and brave, especially during the evacuation of her patron, Queen Maria Carolina, and entourage from Naples to Palermo in 1799, when she seems to have been the only useful, robust soul aboard the transports. She might have made a stalwart Britannia had it been possible for her to become the official Lady Nelson, although there was a profligacy about the couple, a mutual belief in their own publicity and unlimited creditworthiness, that augured ill even had he survived Trafalgar.

Mann wants the reader to lose affection for his subject, but partly because of his use of the Photoplay idiom of Hollywood gossip, I didn't care any the less about Hepburn. Self-deluding and self-eluding, agreed; mannered, unarguably; pliable in relation to a few chosen men - agent Leland Hayward, director John Ford (according to Mann, another mucho macho gay), Howard Hughes, Tracy - at least up to the point when she bicycled away. But she was also original and brave, especially delivering an anti-segregation, anti-censorship speech at a Los Angeles stadium in 1947, two weeks into the House UnAmerican Activities Committee's intervention in Hollywood. She might have made a fine champion for liberal political and sexual values if she could have faced the responsibilities of venerability, but she was a Peter Pan devotee from the era of Teddy Roosevelt, "Jimmy Hepburn" as she called herself when young: a boy who never quite grew up.

Anyway, performance, rather than reality, was both women's metier. Williams responds to the originality of Hamilton's Attitudes. They were a multi-media collage stuck together in the 1780s, derived from her backstage view of the Georgian theatre; from the lewd gestures of bordello "posture women"; from the poses artists encouraged from her as a model; from theatrical coaching in Naples; and from her observation of classical art. What mattered was that with a shawl, a few props, perhaps a child in support, with or without music, she could stage miniature shows with plots to suit the audience - easy classical references for Grand Tourists; penitents and peasant girls to amuse society. She had been taught to sing, and did so with an expressiveness unusual at the time; she danced; but mostly she mimed short playlets in a salon. Hamilton shifted between characters with fluid movements of body and drapery, comedy to tragedy, with the small space and lighting emphasising her face in what must have seemed extreme close-up to those used to the long views of the opera house. She prefigured many changes in the performing arts that began around the end of the French revolution - the beginnings of the romantic ballet, popular melodrama and the almost movie-like Phantasmagoria. The bestselling sketchbooks of her were almost stills from a film - a different way to be seen as female.

Mann hardly responds at all to the originality of Hepburn's performances. He does quote a description of her entrance, in an early stage success, as an amazon, running downstairs on to a platform and leaping from that to the main stage, straight towards the audience - prototype for the athleticism now expected in women performers. But as with all his brief references to her acting (barely five paragraphs in a 600-page book goes to her screentime in The African Queen), it's there to serve his thesis about her discomfort with her feminine self, her covert and overt lesbian relationships, her dislike of sexual intercourse - Mann bangs on about that as if creative or sportive sublimation weren't the more fun most of the time. He's acute about the Tracy-Hepburn films setting her up as bright but tiresome, so that she can be demeaned, and spanked, by Tracy in the last act, but he's not prepared to analyse them to understand how her manner, especially her verbal delivery, offered women viewers different possibilities of being female from contemporaries Joan Crawford or Barbara Stanwyck. (Hepburn's characters appreciate being tertiary-educated, even if she did not.) Those "old married couple" behaviours that Mann deplores in, say, Pat and Mike and Adam's Rib at least propose there might be joint life in the 50 years after the meeting and mating with which the rest of the cinema was, and remains, obsessed.

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