Pattern Recognition's Cayce Pollard is a master trendspotter, a ''coolhunter.'' But instead of eyeing the latest shoe craze, she's hired to trace the source of some film clips that are leaking onto the Web and gaining a cult following bigger than ''The Blair Witch Project'''s. After wending through a network of antique-calculator collectors in London, a mouth breather in Tokyo, and double agents from a German graphic-design firm, Cayce discovers that the hunt is as meaningful as the prey. And readers too may find that her journey to uncover ''the maker'' is more exciting than its final destination. ''Pattern'''s far-out ideas and densely worded sentences bear William Gibson's unmistakable imprimatur; in his eighth outing, the author, who invented the future with ''Neuromancer,'' shows he's just as skilled at seeing the present.