From the author of A Woman's Place (1978, not reviewed) and several children's books: an amusing and inventive twist on the inexhaustively fecund Arthurian legends. Here, the narrator is one of the magical, somewhat fearsome, Fey--the fairylike beings called (erroneously) ``the Good Folk'' by wary villagers. Crompton's version tells of how Niviene, the daughter of the Lady of the Lake, has to leave the forest of the Fey with the magician Merlin in an effort to save King Arthur's Peace--a tale that begins rousingly enough as young Niviene and her friend Elana (whom Niviene suspects of having a Human heart) consider eating the huge, beautiful, drugged Gwenevere, who's been abducted by the Fey Otter Mellias. Merlin and the Lady decree that Gwenevere must be returned, since King Arthur, who saved their forest from the Saxons, himself might invade to avenge his Queen. So Niviene's beloved brother Lugh sets out, disguised as a knight- -and, yes, turns out to be you know who. While Lugh/Lancelot suffers almost Human attachment to that great ninny Gwenevere, Elana dies in a boat of flowers, and Niviene succumbs to a ``strong, handsome, terrified'' Human, none other than Arthur, with baby Bran the result. When little Bran is lost, however, a heartbroken Niviene begins her long search--a quest that will take her and old Merlin to Camelot, the horrifying den of Morgan le Faye, and, after miles of galloping over Britain, to a hermit's hut and the secret of the Holy Grail. Throughout, Niviene, who attempts to douse her Human side, reads auras, turns invisible, and smiles to hide her little pointed teeth. At the close, she'll find Bran (tragically) and witness Arthur's supposed death. Several fairy leagues in sophistication below the Elfin Kingdom tales of Sylvia Townsend Warner (which appeared in the New Yorker in the '60s and '70s); still, this spirited saga is told in lyrically appealing prose--and spates of rhyme. An easy double for young adults.