How ghastly all the Mitfords sound, though of course in real life ha-ha they are ideal.” So wrote Diana Mitford to her sister Nancy.

And of course, in some ways, the six Mitford sisters were unspeakably ghastly. Diana married Oswald Mosley, the British Fascist leader, with the Führer in attendance, and could never quite bring herself to condemn Hitler; her sister Unity was so besotted with Hitler that she shot herself in the head when Britain and Germany went to war; Nancy Mitford was superbly funny, but also capable of astonishing cruelty.

As a clan, they could be snobbish, waspish, charming and brilliant. I have been on the receiving end of the Mitford lash myself. Diana Mosley once reviewed a book I wrote about Elisabeth Nietzsche, the sister of the philosopher Friedrich. Elisabeth was a towering horror, a ferocious anti-Semite and a rabid Nazi who turned Nietzsche into a tool for fascism by publishing the contents of his wastepaper basket after he was too insane to stop her. Diana Mosley, however, thought this icon of Nazism had been “rather brave”, and ticked me off for being unkind to her. It was the sheer condescension of the review that amazed me.

But that is what the Mitfords were like: utterly secure in their privileged certainties, even when those certainties were utterly wrong.

A new book of the correspondence between the six Mitford sisters has just been published: it is 832 pages short, but represents barely 5 per cent of the 12,000 letters the sisters exchanged from the 1920s until 2003. The book reflects two central aspects of British culture and history: an upper class born to rule (and sometimes to sneer), and an epistolary tradition of astonishing richness. Both are now dying: only one deserves to be mourned.

No one will ever write letters like this again. For almost 80 years, the sisters put their thoughts, emotions, experiences and observations down on paper, folded them into an envelope, stuck a stamp on, and consigned them to posterity. The range of people they knew is breathtaking, from Winston Churchill to Lester Piggott, but even more remarkable is the breadth of feeling: here are all the rivalries, jokes, grief, anger, regret and joy of six very different and intertwined lives.

The personal letter is now an endangered species. In a world of electronic conversation and postal strikes, we seldom commit ourselves to paper, by far the most permanent method of preserving the past. Posterity will be unable to find out what we thought and said to each other privately, for the evidence will vanish as surely as memory itself.

The Mitford sisters knew that, by writing to one another so often and so fully, they were investing in history. For all their feigned bafflement at the media interest in their doings, they were avid self-promoters. “A correspondence suivie of a whole family, so rare nowadays, would be gold for your heirs,” Nancy wrote to Diana.

The collected letters (superbly edited by Diana’s daughter-in-law, Charlotte Mosley) are pure gold. In place of the caricatures – Diana the Fascist, Jessica the Communist, Unity the Hitler-lover; Nancy the Novelist; Deborah the Duchess and Pamela the unobtrusive poultry connoisseur – they provide the warp and weft of daily life as only letters can. It does no harm that at least three of the sisters were brilliant writers, and all were natural wits. “We’ve had tea with Hitler and saw all the other sights,” writes Deborah, in the sort of drawled putdown that only a pure-bred upper Englishwoman could muster for an upstart German dictator. Unity’s repulsive enthusiasm for the same subject is equally distinctive: “Poor sweet Führer, he’s having such a dreadful time.” It is impossible not to enjoy the Mitfords, but hard to love them.

There is something cold and spiteful in their contempt for others, particularly those they considered non-U. Amid the tragedies and traumas the brittle stiff-upper-lippery is chilling. Anything may be mocked. “Laughter doesn’t always mean cheerfulness,” wrote Diana.

Often it was a way of hiding, or inflicting, pain. Their combined story is punctuated by deceits and betrayals, the rows and resulting “non-speakers”. At the start of the war, Nancy denounced Diana to the British authorities as a subversive, “an extremely dangerous person”, but never told her what she had done and continued to visit her children while her sister was incarcerated in Holloway. Even their father, Lord Redesdale (“Farve”), ran off with the housekeeper to live on a Scottish island.

“I must admit, ‘The Mitfords’ would madden me if I didn’t chance to be one,” wrote Diana. They were maddening, supremely talented, and endlessly delighted to be members of their own exclusive sorority, with its private language and endless teasing.

The heyday of the Mitford sisters marked the high-water mark of British upper-class dominance. But these wonderful letters also mark the passing of something much more valuable. The sisters deliberately set out to leave a permanent record of who they were, what they thought, and how they related to one another.

To attempt the same thing today would be considered vain. We communicate more, on more levels, than ever before in history, but preserve virtually nothing. To read the Mitfords’ letters is to step into their lives; individually, they were capable of supreme selfishness, but their correspondence was a gesture of generosity to the future.

Unless we can find a way to gather, edit and safeguard our everyday written conversations, future generations will look back on a void of ephemeral e-mails and transitory text messages, and wonder who we were.

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