LORD, LORD, ANOTHER damned, thick, square book about the Mitford sisters arrives from the publishers. Now, why would anyone want to be reading such a thing, given the plethora of Mitfordiana already in the public domain: the multiple biographies and autobiographies, the tottering stack of volumes of letters and memoirs, autobiographical novels, television documentaries, musicals, lines of wholesome grocery products and tasteful ducal knickknackery.

Even the Unknown Mitford Sister, the one without writing talent, extreme political tendencies or a stately home, nearly produced a cookery book (though the publishers turned it down on the grounds that it wasn’t interesting enough – bet they’re kicking themselves now). Enough already with the Mitfords. Can’t we talk about some other family for a change?

Well, we could, of course, but try as we may, it is amazing how the conversation keeps veering back to the Mitfords. Apart from Diana, Princess of Wales, it is hard to think of any public figure (and there are six Mitford sisters, to just the one of her) about whom complete strangers seem to entertain such proprietorial feelings. All Mitford biographies are dogged by this mysterious power to enthral: before your very eyes you see the biographer beginning with brave determination to be all brisk and objective and put the Mitford phenomenon in context – and then falling under the spell and turning in a more-or-less abject piece of hagiography.

Which means that, as with D, P of W, one has a very curious and partial view of the Mitford world, which can appear to have been one long surrealist fantasy of shrieks, bliss and dread. A generic Mitford life might be summarised thus: born; neglected by chilly mother and left in care of sweet nanny until old enough to be hunted with hounds by roaring father on horseback. Eloped to Spain to marry posh but subversive cousin while still a teenager, hotly pursued by Daily Express. Joined Communist or Nazi party, shot self on outbreak of war, but survived to write sprightly novels, keep hens and be painted by Lucian Freud (“dear little Lucy”) while living in exquisite stately home, giving interviews to journalists in remarkable drawly voice and picking A Whiter Shade of Pale as Desert Island Disc. Die at great age, provoking flurry of newspaper photographs of oneself looking lovely with darling Führer/President Kennedy /Maya Angelou in tow. Buried in country churchyard beside the Windrush, where one spent one’s early years.

Colourful as all this is, it is a world away from the home life of most of us and returns us to the question of why we should want to spend £25 to read about it all yet again, this time in The Mitfords: Letters between Six Sisters, a selection from the correspondence between the sisters (there was a brother, Tom, killed at the end of the Second World War, who professed a “constitutional hatred” for letter-writing).

It is certainly curious that, while television documentaries have rendered us intimately familiar with the lives of many tribal groups of whom we have no first-hand experience, from Irish travellers and Indian railway children to the inhabitants of the dwindling Amazon rainforest, we lack almost entirely “useful sidelights” on our own upper classes. Secretive in their habits, they are generally defensive performers in front of the camera, detecting, perhaps, that those on the other side of the lens have their own, not invariably sympathetic, agenda.

Here, then, in this vast volume of selected letters, so unwieldy as to be impossible to read except while sitting bolt upright in a chair – quite useless as a bedside book – which represents, according to its editor, Charlotte Mosley (daughter-in-law of the late Diana Mosley), just 5 per cent of the 12,000 letters the Mitford sisters exchanged (so we can presumably expect another 19 similarly hefty volumes to come along in due course), is an anthropologist’s treasure: a keyhole view of the private rituals of the British upper classes. Not mitigated by biographical interpretation, but the pure ore of that very peculiar world.

It is not just -ologists who will fall avidly on the Mitford archive. Between them, the sisters mustered a range of acquaintance sufficient to fill a dictionary of international biography: Hitler, Churchill, de Gaulle, Kennedy, Macmillan, Profumo, Lytton Strachey, Duncan Grant, Evelyn Waugh, Lucian Freud, Francis Bacon, Queen Elizabeth the Queen Mother, Julie Andrews, Lester Piggott, Brian Sewell, Jerry Hall . . . the private history of power and influence in the 20th century swarms through these letters.

Then, too, there is the singularity of the sibling phenomenon: what other family, apart, perhaps, from the Osmonds, has produced such a treasure of talent in a single generation? Every sister, whether a professional writer or not (and three of the six ended up writing professionally in one way or another) has an extraordinary natural talent for narrative: for observation, reflection, jokes, dialogue and description, and deploys it with unfailing energy, decade after decade, through triumph, grief, violent political difference, protracted estrangement and final illness – until at last only one sister, the Duchess of Devonshire, is left, and the correspondence falls silent after 75 years.

What is more, though there are shared mannerisms of diction, each of the six voices is so distinctive that there is little need for the identifying devices with which each letter is editorially tagged (a quill for Nancy, swastika for Unity, crescent moon for Diana, and so on).

It is melancholy to reflect that the present volume probably represents one of the last great epistolary archives (a fact of which Nancy Mitford was well aware, writing to Diana in 1963 that “a correspondence suivie of a whole family, so rare nowadays, would be gold for your heirs . . .”).

Grief, anger, envy, longing, joy, amusement, boredom – all the private emotions so vividly pinned and preserved in these remarkable letters – now vanish into the electronic ether of text, phone or e-mail, so generations to come will never know what we felt or how we told our feelings.

Of that, where the Mitfords are concerned, one is never in the slightest doubt. Love or loathe the idea of them, there is no denying the vivid immediacy of their polyphonic voices in this remarkable volume, the editing of which by Charlotte Mosley is distinguished by its ideal mixture of tact, efficiency and unobtrusiveness.

Buy The Mitfords: Letters Between Six Sisters edited by Charlotte Mosley
Fourth Estate, £25, 832pp

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