Much celebrated for the inventiveness of his imagination, West (Terrestrials) turns to his own life for the plot of his 18th book. The name he gives his protagonist, Ariada Mencken--an anagram for Diane Ackerman, West's wife--establishes first off how thin the fictive veil draped over this fictional memoir will be. Speaking through the voice of his narrator, West eulogizes poet/nature essayist Ackerman's black-haired beauty and says that, at 22, hers was ""the best poetry [he'd] seen since Dylan Thomas."" The narrator's nickname for her, Swan, is a hieroglyph of her features: her interest in flight, her poetic gifts, her unconscious elegance. Against the backdrop of their academic, writerly lives--time is marked by such and such a teaching job, this or that novel or collection of poems published--another theme takes shape. Space travel begins to fascinate the two when they meet Raoul Bunsen (presumably Carl Sagan) at Coriolis (Cornell) University. He introduces them to the NASA savants who, in the '70s, planned the Viking and Voyager flights. Rather touching, though a bit too pleased with itself, the book unravels in its latter third as West obsessively chronicles the details of Swan's flying lessons, his books, their parties. One begins to feel that West will not relent till he's shown you every last snapshot in his wallet. (Feb.) FYI: West recently received the Chevalier of the Order of Arts and Letters from the French government.