It's hard to believe this is the first full biography of Nancy Mitford, that slightly tarnished ""Bright Young Thing"" who, with dear Gertie and Noel, Linda and Cole, Wallie and David, has racketed through so many reminiscences of the 20's, 30's and 40's. By this time, her exploits are as familiar as the strains of ""I'll See You Again."" Considering the dÃ‰passÃ‰ quality of the material, Mitford has been fortunate in her biographer. Hastings is apparently young enough to be charmed by her subject and is kinder to her than many will feel Nancy ever deserved. What Hastings terms Nancy's ""teasing,"" for example, the less charitable might dub bitchiness. What is called ""self-assurance"" others might construe as snobbism, pure and simple. Hastings is able, seemingly without too much embarrassment, to report on ""that hilarious evening when Marigold (a traveling companion of Nancy's) was prevailed upon to throw her asparagus stalk over her shoulder at dinner."" She is able, furthermore, to recite the endless nicknames (""Decca,"" ""Sooze,"" ""Muv,"" ""Farve"") and baby talk (""delishwish,"" ""tinkety tonk,"" ""poodle pie"") with which Nancy dotted her conversations and letters, without gagging. Ms. Hastings is a wonder. The author turns a slightly more jaundiced eye on one important area of Nancy's life--her deplorable taste in men. And it's here that the biography offers something more than the usual rehash of previously published anecdotes. For all her much vaunted ""worldliness,"" Mitford was remarkably naive in her romantic entanglements. For years she pursued Hamish St. Clair Erskine, a fey young Britisher who a few years before had had an affair with Nancy's brother Tom. Her marriage to Peter Rodd wasn't much more successful--long separations, violent scenes, eventual indifference. Even her longstanding affair with Frenchman Gaston Palewski finally came to naught when he married another woman--but not until he'd been used as a model for ""Fabrice, duc de Sauveterre"" in Mitford's immensely successful novel, The Pursuit of Love. In these revelations, Hastings is able to add a humanizing dimension to her subject's life. It's more than a little welcome. The book, we are assured, was written with ""the fullest cooperation of Nancy's family and friends."" One eagerly awaits a companion volume in which ""acquaintances and enemies"" are consulted.