Decca: the Letters of Jessica Mitford, Ed. Peter Y Sussman

No doubt about it: Jessica Mitford (1917-1996) had one hell of a life. Whether as a precocious child sharing a bedroom with her proto-Fascist sister Unity Valkyrie in Lord and Lady Redesdale's Cotswolds nursery, eloping to the Spanish Civil War with her cousin Esmond Romilly, setting up as a US civil-rights activist with her second husband Bob Treuhaft, or embarking on a fresh career as a crusading journalist, she combined the ancestral taste for high society with an attachment to every good brave cause that her adoptive country had to offer. Smart radical bohemia knew no finer exponent. And yet by far the most interesting thing about Decca, as in anything written by any one of the six Mitford sisters, is her voice.

Jessica herself denied that it existed. A letter sent in her late seventies to Charlotte Mosley, who was editing the correspondence of her sister Nancy, runs: "About the alleged 'Mitford voice' - don't you think that's a pure invention?" On the contrary, the most remarkable thing about Jessica's style, even half a century after her self-exile from England, is how consistently it draws on the deep reservoir of familial experience. Her immersion in US society went only so far; there remained the amused, amusing and occasionally downright glacial figure of the English aristocrat en vacances, capable of asking the young Hillary Rodham, of her newborn daughter, "Was she conceived in Victoria station or Chelsea?"

As for the Mitford voice itself, no simple definition applies. It is not the standard upper-class shriek of the inter-war years - the voice of Evelyn Waugh's dialogue, say, in which the insignificant is wantonly exaggerated and the significant downplayed.

Rather, it creates a kind of linguistic space where the irony is so super-subtle that it sometimes escapes detection, where anything can be said, or conspicuously not said, and where the people one loves most fiercely can be ripped into pieces without ever compromising the ties of kinship. "Madame is awfully nice," the teenage pension-frequenter reported to "Muv" from Paris in 1934, "only rather dirty & probably slightly dishonest, like most French people."

What Decca really thought about Madame is lost forever in the murky compact between writer and recipient. It is the same in 1942 when, with Esmond about to vanish on a Canadian RAF flight, she addresses Muv on the subject of her beloved brother Tom. "I hope old Tud will be home soon, I suppose it is awfully worrying to have him out there. Is he a general or anything yet?"

Peter Sussman's absorbing compilation is full of this kind of thing. Flippancy and seriousness uncomfortably mingle, cultures riotously clash. A resumé of lunch with "rather a dull girl called Ladybird Johnson", wife of the future US president Lyndon B Johnson, prompted Lady Redesdale to wonder: "Who is Lady Bird? I looked her up in the peerage but could find no trace." America, though suspicious of Jessica's Communist Party membership and her work for the Civil Rights Congress, was, in a phrase that could have come straight out of Nancy's novels, "absolute heaven".

Curiously enough, all this worked in Jessica's favour. Most of the English high-lifers who went Left in the 1930s found themselves fatally compromised by an inability to jettison the social or aesthetic baggage of their earlier lives. Ultimately, the Ritz bar won out over the protest march. What an American friend called her "concrete upper lip" was a positive advantage in Jessica's civil-rights work. While her despatches from the US political and social front-line are always fascinating - see in particular the account of a lynch mob's attack on a church in which she and fellow-activists were entombed - the best parts of the book take in her deeply ambivalent relationships with her sisters.

The years of intermittent niggling ("not on speakers" or, worse, "not on writers") reach an icy crescendo in the communal mid-1970s spat about David Pryce-Jones's biography of Unity. "I'm absolutely enraged by your foul letter," Jessica informs Pamela, who had accused her of swiping the Chatsworth photo album.

A mammoth volume of the letters exchanged between all six of them is promised for next year. Doubtless over-exposure will see the Mitfords go the same way as Bloomsbury - turned into a raree show of upmarket gossip - but this reviewer can hardly wait.

DJ Taylor's 'Bright Young People: The Rise and Fall of a Generation' appears this year from Chatto & Windus