THIS IS an urbane family saga for those who belong to what Jonathan Guinness here calls "the talking classes," which means most of us. The Mitford girls have been talked about ever since Nancy, the eldest, was born in 1904; she became a novelist and a francophile. Diana married Sir Oswald Mosley, the fascist; Unity idolized Adolf Hitler; Jessica became passionately left-wing. Only Pam and Debo opted out of notoriety, though Debo is distinguished as the Duchess of Devonshire and chatelaine of Chatsworth.

There is already a full biography of Unity, and a life of Nancy by Selina Hastings is in preparation. Nancy's autobiographical novels and the memoirs of several other sisters have contributed to the Mitford myhology and the Mitford industry, to which The House of Mitford is a hefty addition. Jonathan Guinness is the son of Lady Diana Mosley's first marriage, and his daughter Catherine has helped him with the research.

The author is at pains to point out that he is taking the Mitford story seriously; the society gossip-column way in which their activities have been chronicled (there has even been a musical) has trivialized them and enabled "writers of the pedestrian sort" to dismiss their significance. He sets the Mitford girls in a context of English social history by suggesting that they were reacting against the old imperial certainties embodied by their forbears. Even if their parents were eccentric, they were the landed gentry, conservative and church-going. His book begins with extended studies of the two grandfathers -- Bertie (pronounced Bartie) Mitford, landowner, MP and the first Lord Redesdale, and Thomas Gibson Bowles, magazine proprietor, sailor and practical joker. More than 200 pages of fily history must be faced before the six pretty Mitford girls and their brother Tom make their appearances.

Elsewhere the author playfully deflects the reader from the serious approach that he himself has proposed: "The trouble is that a number of rather solemn people have found occasion to write about the Mitfords." His own narrative is desolemnized by quotations from the poets, personal asides about Paris traffic and modern architecture and facetious references to grave events -- "a bit of bother" in Hungary in 1956. He does his best to convey the Mitford "manner" -- the turns of phrase, cult words, funny voices, intonations and private slang that evolve to some extent in any large articulate family and which, with the Mitfords, was raised to a high theatrical level. He regrets that so much witty talk has gone unrecorded; but this may be God's mercy on the Mitfords, since what is remembered dies on the page. The following, for example, is given as a sample of Nancy's characteristically hyperbolic mode: "During the day Evelyn Waugh took Nancy to lunch at the Hyde Park Hotel; there was toast on the table in little silver racks. 'Oh!' she exclaimed in tones of rapture. 'Toast!'" End of story. Nancy was the keenest of the six on practical jokes, Guinness relates, then consigns to mythology, several sadistic japes attributed to the Mitford girls -- while resurrecting others, funnier but no less callous. Decca (Jessica) "used to shake her father's elbow as he was drinking his tea, to give him palsy practice for when he was old."

DECCA became a Communist sympathizer, and with her first husband, Esmond Romilly (a nephew of Churchill's wife), conducted "a guerrilla war against their own class." This consisted, while staying with a socialist peer, of "keeping the servants up all night ringing for sandwiches, tea, rum or cigars." The nonsense of this needs no underlining; but neither does the fact that the author is not keen on this aunt, a prominent left- wing writer who has worked in the U.S. now for many years. She is the only one of the surviving Mitford sisters who did not cooperate with the Guinnesses over this book. Guinness is on the far right of the British Conservative Party, and while he gives due weight to every variety of Mitford political attitudes, a condescending note is discernible on the subject of left-wing "waffle." He observes how the Mitford girls tended always to "link personal love to a general cause"; and when Diana fell in love with Oswald Mosley, both the man and his ideas, "it was the passion of Juliet and at the same time it was the conversion of St. Paul" -- asinine analogies for a married woman with two children, falling for the leader of the British Fascists.

Not unfairly, Guinness claims that here "we have to picture Fascism not as it appears today but as it appeared in 1933," even though he is mildly critical elsewhere of Nancy's "drawing-roomhistory" and of the way she reconciled her social class values and her theoretical socialism by suggesting that "what was wrong at one period of history may be right at another." In any case Diana and Unity ("I want everyone to know that I am a Jew hater") stuck to Hitler in the years that followed; Diana's wedding reception after her marrige to Mosley was in Dr. Goebbels' house. Unity's extremism was on the grand scale that combines moral idiocy with tragedy: on the outbreak of war she made an unsuccessful suicide attempt that left her brain-damaged.

The Mosleys, however, soldiered on. Guinness crowns his discussion of this uneasy topic by drawing a distinction between Mosley's "hostility to the Jews," apparently on political grounds, and Hitler's "anti-Semitism"; there was a "vital difference" between Mosley and Hitler which it is "only fair"to acknowledge. This is it: "For Hitler, a Jew, however well-intentioned, could never fully renounce his Jewishness. For Mosley he could do so." Words fail me . . . as they never failed the Mitford girls nor, on this showing, their descendants. The Mitfords, Guinness says, like to make "story" out of everything, however terrible or wonderful. His book bears this out, displaying matters of life and love and death, their own and others', as an exercise in performance art.