Lady Diana Cooper swoops like a proud butterfly through 20th-century Britain's high life and uncommon culture--in Evelyn Waugh's diaries and letters, in other writers' novels, in everybody's memoirs (including her own three crisp volumes, 1958-1960). Here, however, the butterfly is most unflatteringly pinned to the center of a long, adoring, increasingly off-putting biography: a cascade of trivia unredeemed (as it might perhaps have been) by bright stylishness or shrewd socio-historical texture. Ziegler doggedly follows his subject--from turn-of-the-century childhood as a Duke's daughter (actually her real father was dashing Harry Cust, mum's lover) to celebrity as a dancing, willful, incredibly sought-after beauty. He evaluates her major suitors and smiles at her foibles, from not teaming to spell (""an accomplishment which she considered at the best otiose, at the worst slightly common"") to morphine, anti-Semitism, and her ""shameless"" exploitation of rich or powerful friends. He's sure that she wasn't, as charged, unfeeling (""at the worst she was guilty of a kind of cowardice""). He details her WW I nursing work and her ""triumphant"" postwar marriage to highly un-eligible Duff Cooper (her great, impossible love was for married Raymond Asquith)--a writer/politician whose philandering non-sensual Diana accommodated nicely: ""Sometimes she seemed almost to procure for him."" And, though things pick up a bit when money-needy Diana tours as a non-speaking sensation circa 1930 in Max Reinhardt's The Miracle, tedium sets in again with the accounts of Diana's years as a political wife, a WW II farmer, an infrequent adulteress, an organizer of ""hectic motorised treasure-hunts,"" a traveler in Asia, an ambassador's wife in Algiers and Paris, and legendary senior citizen (now 88). Admittedly, Ziegler does find a smidgin of character development by 1943: Diana becomes a little more tolerant and socially responsible; she bravely overcomes a tendency to melancholia. But, with too much of Diana herself and too little of the creative or important people around her, this momentum-less chronicle only emphasizes the fragile nature of Lady D.'s celebrity, risking a who-cares? reaction on pass after pass. And Ziegler's literate but uninspired (and sometimes cliched) prose never manages to turn this into an infectious, gossamer period-piece. Only the most fervent, nostalgic Anglophiles, then, will want to share Ziegler's traipse through every bit of Diana-ana; other, less fanatical sorts should stick with the standard sources--if they want to preserve their fond picture of that elegant, irresistible madcap.