20,000 YEARS OF FASHION by Francois Boucher (Harry N. Abrams: $39.95; 459 pp.). This is a splendidly illustrated history of fashion, an updated version of a classic in its field. It suffers only from a stolid (and sometimes awkwardly translated) text--a text that treats Bronze-Age relics, Cretan costume, and the frou-frou of the Edwardian demi-mondaine in the same curatorial tone. Still, the illustrations are ample, well chosen and invariably interesting--especially in the chapters relating to the 18th and 19th centuries, for whose gorgeous costumes Francois Boucher clearly has a weakness. (The author was Honorary Curator of the Musee Carnavalet, and Director of the French Center for Costume Studies, in Paris.) The last part, dealing with contemporary clothes (1960-'83), does not fare so well: The illustrations are disappointing, not to say perplexing. (What they do show, however, is how distant, strict and anoretic the looks of the past several decades suddenly appear--Future Shock, perhaps, or the recent advent of Christian Lacroix and his beruffled fantasies?) Despite its flaws, this is a sound, if not scintillating reference book for fashion scholars, or for those who are just fashionably curious.