Sex and Philosophy: Rethinking de Beauvoir and Sartre, By Edward Fullbrook and Kate FullbrookA Dangerous Liaison, By Carole Seymour-Jones

Since the deaths of Jean-Paul Sartre and Simone de Beauvoir in 1980 and 1986 respectively, their personal reputations have taken a battering. The publication of Beauvoir's letters in 1990 revealed the extent of this radical couple's manipulation of young women – and men – for their own sexual, and occasionally intellectual, needs. Subsequently, her love letters to Nelson Algren appeared in 1997, and a significant love interest, omitted from the journals, with Jacques-Laurent Bost, emerged in 2004.

What Sartre and Beauvoir meant to each other has become a serious question partly because of this gradual secretion of contradictory pieces of information. But the personal life has also taken precedence because Sartre and his works have fallen out of fashion. It was Sartre who declared himself a mouthpiece for the Resistance after the war when he did little during it; Sartre who was the real "fellow-traveller" during the Cold War, denying Stalin's purges and falling in love with a KGB agent. And it was Sartre to whom existential philosophy owed its existence, a philosophy since eclipsed by post-modern theories.

Perhaps not surprisingly then, it's Beauvoir whom biographers and critics believe they can more readily reclaim; her name comes first in both books' subtitles (as it did in Hazel Rowley's 2005 portrait of the couple). Both Seymour-Jones and the Fullbrooks make much of Sartre's "borrowing" from Beauvoir's first novel, She Came to Stay, for his major philosophical work, Being and Nothingness, arguing that Beauvoir was the real philosopher of the pair.

But why reclaim the "real" philosopher when that very philosophy has gone out of fashion? Why is grooming schoolgirls for sexual relationships with your male partner any less reprehensible than falling for Russian spies? Both Sartre and Beauvoir's reputations vie with each other in their less salubrious aspects but it's the pitiable, pathetic image of a late middle-aged Sartre, hastening round the residences of the various women whose rents and bills he's paying, that lingers, and is set against the glamorous Beauvoir, doyenne of the women's movement, enjoying love where she can find it and staying loyal to the end.

Seymour-Jones begins her excellent biography with the familiar: Beauvoir's family falling from gentility into hard times and Sartre's excessive closeness to his mother after the death of his father. Both were scarred by their early experiences; both were brilliant scholars; both were attracted to philosophy, yet wanted fame as novelists. When they met at the Ecole Normale in 1929, Beauvoir was with a friend of Sartre's, René Maheu, who would give her her nickname, "Beaver". Sartre defeated her in their final oral exams, coming first to her second, but she beat him when he proposed marriage – she rejected him. Instead they formed their special liaison, their "essential" love to which other relationships would be "contingent".

The myth Sartre and Beauvoir perpetuated is that nobody came close to their love for each other, but Beauvoir's letters told a different story. They reveal her affair with one of her pupils, Olga Kosackiewicz, whom she later encouraged to sleep with Sartre, and her passion for Jacques-Laurent Bost, one of Sartre's students. They kept their relationship secret from Olga, later Bost's girlfriend. Bost became a contender for first place in Beauvoir's heart, and some of her relationships with women came closer than she let on.

This kind of sexual merry-go-round makes great copy now, but it was deadly serious at the time, and often damaging to the women involved. (Sartre and Beauvoir did later regret their manipulation of those young women.) Erotically Beauvoir had the edge over Sartre, as she was attracted to both men and women, so nowhere, Seymour-Jones argues, was she ever the "victim" in their relationship. Sartre lied to her on many occasions, but she lied to him, too. Her faith in him has troubled feminists ever since The Second Sex became a landmark text, but Seymour-Jones makes it clear that it gave her freedom. She could do what she wanted, even if it came at a high price (only a very strong woman could have withstood the regular emotional fall-out of their way of life).

The Fullbrooks, in their scholarly but accessible overview of Sartre and Beauvoir's philosophical work, reiterate their contention that Beauvoir's thoughts, mapped out in her first novel, were plagiarised by Sartre, and go into some detail to prove it. Their Beauvoir is a victim of a patriarchal society that can't accept women as philosophers, and of a shabby male partner, who couldn't admit the love of his life had a better brain than him. Beauvoir's reluctance to claim credit for ideas central to Being and Nothingness is a consequence, they say, of her first works of fiction being rejected, just as Sartre's literary reputation went stellar in 1938, with the publication of Nausea. After that, in the public's mind, Beauvoir always came second, just as she had in the oral exam. Nowadays, they argue, we all know better.

The aim of both these books is not really to reveal startling new pieces of information, although Seymour-Jones has worked hard to uncover the full extent of Sartre's affair with Lena Zonina, the KGB spy/translator. Rather, something else is going on here – a cultural shift to place Simone de Beauvoir centre stage. Both books want to claim her as instigator, negotiator, philosopher, free woman, but neither particularly wants to dwell on the unpleasantness of her relationships with under-age girls (Seymour-Jones does go so far as to use the p-word), and neither wants to question her capability for loyalty to anyone, even to herself.

In that respect, Beauvoir will always be a compromised sort of heroine, even to the most ardent fan. But that's better perhaps, than simply being the sidekick, which is all poor old Sartre manages to achieve here.