Susan Sheehan won the Pulitzer Prize in general nonfiction for her book “Is There No Place on Earth for Me?”

The Mitford sisters may be the most written-about girl group in history. Nancy, Pamela, Diana, Unity, Jessica and Deborah: The flamboyant British aristocrats have kept tabloid journalists and biographers scribbling away for nearly a century. Between their high-profile trysts, questionable politics and literary output, it’s no wonder they’ve been the source of fascination.

Mary S. Lovell published “The Sisters,” a definitive biography of the Mitfords, in 2001. Now comes “The Six” by Laura Thompson, author of books on subjects as diverse as greyhound dog racing, Agatha Christie and the Lucan Affair and, most recently, a biography of Nancy Mitford. The question is whether we need another book about the Mitfords.

Thompson herself notes that she’s treading a well-worn path. “Familiarity is undoubtedly an issue,” she writes in the introduction, but she asks that readers “look afresh at the familiar and consider. These girls are prize exhibits in a Museum of Englishness. What they represent is complex, although their image has divine simplicity. And whatever one’s opinion of what they represent, it is impossible, in truth, to find them boring.”

True enough, the sisters are rarely boring — and in a narrative that contains some stylish prose, Thompson dwells on their quirky charms. Readers will need the Mitford family tree that appears at the front of the book, however, as Thompson leapfrogs through the saga of this infamous sisterhood.

Deborah Mitford was the youngest of six aristocratic sisters who, in the early 20th century, consorted with British and American royalty. Some even befriended Adolf Hitler. (Associated Press)

The Mitford parents, David and Sydney, Lord and Lady Redesdale, were minor British aristocrats. The daughters all made their debuts, despite the fact that Muv and Farve (this is a family of countless nicknames) kept having to sell property to pay for presentations at court, cruises, nannies and governesses. The daughters were expected simply to find suitable husbands and breed, but instead of settling down quietly, five of the six achieved notoriety.

First there’s Nancy Mitford, the eldest, born in 1904 and the best-known; she was the author of popular and well-regarded semi-autobiographical novels (“The Pursuit of Love” and “Love in a Cold Climate”) and biographies such as “The Sun King.” Thompson is generous — one could say overly generous — to Nancy’s novelistic skills, describing “Love in a Cold Climate” as her “second masterpiece.” Upon its publication, Thompson adds, Nancy’s status as an author “reached the exalted position for which writers pray, in which every single thing that they publish is received with rapture and no failure can really touch them.” Her social life, too, “was a glitterball whirl.”

Diana, the beauty in a good-looking family, married in 1929, had two children, then left her first husband, the Hon. Bryan Guinness (heir to the brewery fortune) for Sir Oswald Mosley, the married, philandering founder of the British Union of Fascists. The Mosleys, who married in Joseph Goebbels’s drawing room in Berlin in 1936 in a ceremony attended by Adolf Hitler , were imprisoned during the Second World War. What Diana would learn, 40 years later, was that Nancy, to whom fascism was “a disease,” had done her best to ensure that Diana would be imprisoned and, subsequently, kept that way instead of being released under house arrest. “This was the central relationship of the Mitford girls, this push-and-pull between Nancy and Diana,” Thompson observes.

Unity Valkyrie Mitford, an ardent fascist, met Hitler in 1933 and moved to Munich a year later. Hitler took pleasure in her company. Unity attempted suicide the day Britain declared war on Germany by shooting herself in the head, but she survived the bullet. The man she called “blissful Fuhrer” had her flown to neutral Switzerland. After Unity’s return to Britain, Muv took care of this brain-damaged daughter until she died of meningitis in 1948.

Pamela Mitford, a fascist sympathizer, became the second wife of a renowned and rich physicist, who married three more times after divorcing her. This so-called quiet sister lived in the country and, like Unity, didn’t write books. (The only son in the family, Thomas Mitford, who also had fascist leanings, pursued a military career. He declined to fight Germany, preferring to fight the Japanese, and was killed in Burma in 1945 .)

"The Six: The Lives of the Mitford Sisters," by Laura Thompson (St. Martin's)

Jessica, a fervent left-winger, eloped with her second cousin, Esmond Romilly, Winston Churchill’s nephew by marriage, to Bilbao; he had fought with the communists in the Spanish Civil War. The couple subsequently emigrated to America, where Romilly volunteered for the Royal Canadian Air Force and was killed in action in 1941. Jessica remained in America. Two years later she married Robert Treuhaft, a partner in a radical law firm. Mitford and Treuhaft were active communists until 1958. She went on to write “The American Way of Death,” a best-selling exposé on the high cost of funerals in her new land. One of the odder passages in “The Six” pertains to Jessica’s politics. “Jessica’s extremism is more acceptable to history than that of her sisters,” Thompson writes. “Such is the luck of the left.”

The youngest sister, Deborah, born in 1920, married Andrew Cavendish, who became the 11th Duke of Devonshire after the wartime death of his older brother, Billy Hartington, the husband of Kathleen Kennedy. The duchess renovated Chatsworth, the Devonshire ancestral seat, and turned it into a thriving enterprise. She consorted with British royalty and American royalty — she attended President John Kennedy’s inauguration and his funeral — and wrote a number of books, including a featherweight memoir just before her death in 2014.

“The Six” includes thousands of facts about the Mitford family, but Thompson offers few clear opinions of her subjects. She ought to have done so. Never mind their popularity, most of the Mitfords were unlikable. Their politics were appalling. There is no letting them off the Hitler hook. The most sympathetic figure in the book is Diana’s first husband, Bryan Guinness, who offered the adulterous Diana the necessary false evidence of his infidelity so she could procure a divorce. Thanks to his generosity, she moved with their two sons to Eaton Square and lived in luxury until Mosley’s young wife died. After a few more dalliances for Mosley and two abortions for Diana, they married.

The book offers so much material — too much, perhaps, and much of it redundant. “The Six” is fine for readers new to the Mitfords, but the definitive biography remains “The Sisters.”

The Lives of the Mitford Sisters

By Laura Thompson

St. Martin’s.
385 pp. $29.99