Tolstoy believed that all unhappy families are unhappy in their own way; if he had known the Mitfords, he might have reconsidered. The Mitfords, of course, were no ordinary sisters, and their problems were not the kind most families have to deal with. Yet reading the “The Mitfords: Letters Between Six Sisters” (Harper Collins, 834 pages, $39.95), the giant new selection from nine decades’ worth of their correspondence, it becomes clear that family dynamics are much the same whether the family is made up of peasants or peeresses. Rich, famous, and frequently horrible as they were, the Mitford sisters too fought about who their parents loved most, who teased whom the worst during their childhood, and who should be in charge of the family photo album. “I must admit ‘the Mitfords’ would madden ME if I didn’t chance to be one,” wrote Diana, the most notorious of the six, in 1985. Reading these letters helps to make them, if not less maddening, at least more human.

The humanity needs rescuing, because it has been encrusted by nearly 80 years’ worth of mythmaking. For much of the 20th century, the Mitford sisters — Nancy, Pamela, Diana, Unity, Jessica, and Deborah, born between 1904 and 1920 — regularly found themselves on the front pages of newspapers, and seldom for admirable reasons. In the 1930s, when they were beautiful young socialites, the Mitfords’ scandals were romantic ones — their elopements, divorces, and extramarital affairs were as eagerly chronicled as those of Paris Hilton or Angelina Jolie are today. Their mother, the retiring Lady Redesdale, told them, “Whenever I see a headline beginning with ‘Peer’s Daughter,’ I know one of you children has been in trouble.”

With the approach of the Second World War, the Mitford saga took a darker turn, and turned the sisters’ social notoriety into political infamy. Unity, whose mental health seems to have been frail, developed a bizarre crush on Adolf Hitler, and managed by assiduous stalking to worm her way into the Führer’s inner circle. When England and Germany went to war, Unity was unable to reconcile her conflicting loyalties, and she shot herself in the head. But the bullet succeeded only in crippling her, and she lived until 1948 in a state of childlike imbecility.

More sinister, because more intelligent and capable, was Diana, the beauty of the family, who already as a teenager moved in the highest literary and artistic circles. Evelyn Waugh dedicated “Vile Bodies” to Diana and her first husband, Bryan Guinness, and used them as models for his brittle, glamorous Bright Young Things. In 1932, however, Diana fell in love with Sir Oswald Mosley, the sinister leader of the British Union of Fascists, and began to live openly as his mistress. The couple was married a few years later, at the home of Joseph and Magda Goebbels, with Hitler in attendance.

With friends like those, it is no wonder that during wartime the Mosleys were interned as potential traitors. After 1945 they spent much of their time in effective exile in France and Ireland, as Mosley tried desperately to regain the limelight he had briefly enjoyed at the beginning of his career. But Diana never ceased to stand by her gifted, hateful man, and to the end of her life enjoyed discoursing on Hitler’s genius — even though the only Mitford brother, Tom, was killed fighting for the Allies in the Second World War. Jessica, the next-to-youngest sister, rebelled against her aristocratic upbringing in another fashion, eloping at age 17 with the glamorous young communist Esmond Romilly. The couple went to America, and after the outbreak of war, when Romilly enlisted in the Royal Air Force and was killed, Jessica decided to stay. She married a fellow communist, a lawyer named Robert Treuhaft, and moved to Oakland, Calif., where she combined pioneering civil-rights activism with loud adulation of the Soviet Union. (She named her son Nicholas Tito, after Lenin and the Communist dictator of Yugoslavia.) In the late 1950s, Jessica finally quit the party — though she never quite lost her nostalgia for it — but she remained a crusader. Her polemic against the funeral industry, “The American Way of Death,” made her famous — she even had a type of cheap coffin named after her. The oldest sister, Nancy, and the youngest, Deborah, steered clear of politics, but they too found ways of staying in the public eye. Nancy achieved literary fame in 1945 with her novel “The Pursuit of Love,” based in large part on the Mitfords’ own childhood. After the war she moved to France, where she conducted a decades-long affair with Gaston Palewski, a high-ranking member of the de Gaulle government. Deborah took the most respectable path of all, marrying the heir to a dukedom and eventually becoming the duchess of Devonshire. Pamela, who devoted herself to farming and animal breeding, seems to be the only Mitford sister whose head was not turned by the family’s notoriety.

For by the time they were middle-aged, the other surviving Mitfords had turned the cultivation of that myth into a full-time job. Nancy got the ball rolling with “The Pursuit of Love”; Jessica followed with a memoir, “Hons and Rebels”; then Diana had a turn with her unrepentant autobiography, “A Life of Contrasts.” Decade after decade, the sisters were interviewed, profiled, filmed, biographized together and separately — there was even a musical about them. Indeed, if you are looking for an introduction to the Mitfords’ lives and legends, there are better places to start than “The Mitfords,” despite the definitive-sounding title. Mary S. Lovell’s group biography “The Sisters” offers a better summary of why the Mitfords mattered. Nor are these family letters necessarily the best specimens of the sisters’ correspondence: Nancy’s brutal wit is better displayed in “The Letters of Nancy Mitford and Evelyn Waugh,” and Jessica’s causes are more passionately pursued in “Decca,” the collection of her letters that was published just last year.

Writing to one another, it is clear, the sisters did not try so hard to impress, or to live up to the public roles they had invented. The appeal of “The Mitfords,” rather, lies in its casual intimacy. Practically every letter is full of private jokes, nicknames, and nursery lingo, to which the sisters return as to the “Hons’ Cupboard,” the secret childhood hiding place that Nancy and Jessica each wrote about. No one is ever addressed by her first name in these letters: the formidable Diana is “Honks,” the spiky Jessica is “Decca,” the grand duchess of Devonshire is “Henderson” or “9” (for her alleged mental age). A typical letter, written by Deborah in 1980, begins: “Honks & Sir O & Woman & I went to see the film of us re Nancy last week, also Jonathan & Middy …. You will SCREAM.” The effect is both cozy and airless, leaving the letters redolent of what W.H. Auden called “ruthless / Verbal in-fighting as it is taught / In Protestant rectories upon drizzling / Sunday afternoons.”

The other source of the Mitfords’ dialect is the slang of the 1920s, which Waugh captured so perfectly in his early novels — bright, deadening, and full of exaggeration, the drawl of a postwar generation that expressed emotion by parodying it. The effect of this style is to make even genuine sentiments sound insincere, as when Nancy consoles Pamela on the breakup of her engagement: “Oh I am so sorry how beastly for you poor darling. … Best love & don’t be too miserable, I am, dreadfully, about it but one must make the best of things.” That aristocratic heartlessness is the keynote of Nancy’s letters in particular, but it is heard from each sister at times, and it becomes more obnoxious the more serious the subject at issue.

Take, for instance, Unity’s letter to Jessica in 1937, explaining why her Nazism shouldn’t stop her from being friends with Jessica’s Communist husband: “My attitude to Esmond is as follows — and I rather expect his to be the same. I naturally wouldn’t hesitate to shoot him if it was necessary for my cause, and I should expect him to do the same to me. But in the meanwhile, as that isn’t necessary, I don’t see why we shouldn’t be quite good friends, do you.” It is like listening to a sociopathic Lady Bracknell.

Not until the 1960s, as the sisters enter middle age and put the more florid stages of their careers behind them, do the letters stop being so insistently amusing and begin to make room for introspection. Inevitably, with the passage of time and the death of “Muv” and “Farve,” they turn back to their childhood and try to sort out its rights and wrongs. What gives this process a special piquancy, in the Mitfords’ case, is that the sorting-out takes place in public and between hard covers, so that one sister’s memory is another sister’s libel.

The main antagonists in these years were Jessica and Diana, whose politics already made them bitter enemies, and who never wrote to one another. This left Deborah as the family umpire when David Pryce-Jones’s biography of Unity Mitford appeared in 1976. Jessica cooperated with the exposé, while the other sisters refused, hoping to protect Unity’s reputation. The usually detached Pamela wrote to Jessica attacking “the worm’s book,” and implying that Jessica had stolen a family album to hand over to Mr. Pryce-Jones. Jessica, always self-righteous and quick to anger, sent a withering reply that begins, “I was absolutely enraged by your foul letter.” (Typically, however, even this missive is addressed to “Woman,” Pamela’s old nickname.) Diana, meanwhile, wrote self-righteously to Deborah, “I’ve seldom been more depressed by anything than by Decca’s letter,” adding a barb about her “horrid little husband” (who, as Diana never forgot, was Jewish).

The whole exchange is as bitter and undignified as any family feud. Yet in the aftermath, the surviving sisters seemed to be able to write more openly about their feelings, and to compare their conflicting memories of their childhood. They are united, too, by an increasing feeling of not belonging in the contemporary world: They struggle with fax machines and dishwashers, and comment on “Antiques Roadshow” and the death of Princess Diana. (“How strange it is, this adoration & beatification of the princess,” Deborah writes; “‘they’ have no idea of the other side. She was mad of course.”) Once Jessica dies in 1996, leaving Diana and Deborah to keep the flame, the Mitford phenomenon begins to seem venerable from its sheer persistence. Even now, the Duchess of Devonshire survives to provide a link with the distant era when the Mitford sisters were gossiped about on two continents. Just last month, at the age of 87, she went on the BBC to flog “The Mitfords” — a reminder that of all the sisters’ talents, the greatest was the talent for publicity.

akirsch@nysun.com