THE FIRST COLLABORATION between the two masters of "cyberpunk" science fiction, The Difference Engine (Bantam Spectra, 429 pages, $24.95 cloth) is the long-awaited, encyclopedic novel of an alternative Victorian England in which Charles Babbage`s successful "difference engine` brings about the computer revolution 100 years too early. In William Gibson and Bruce Sterling`s finely detailed historical otherworld, steamdriven computers keep tabs on all of Britain`s citizens, Karl Marx is the leader of a revolutionary commune in Manhattan, and the Romantic movement never begins - partly because Lord Byron is the prime minister of Britain. Taken simply as a convoluted thriller, the novel works quite well; imagine Wilkie Collins speeded up courtesy of the contemporary streetwise sensibilities familiar to fans of William Gibson. But there is a great deal of space devoted to other complexities as well, since the novel is also Preoccupied with information theory. The introduction of computers into another point in history isn`t just a clever piece of speculation, it`s a proving ground for the ideas that Gibson`s co-author, Bruce Sterling, has smoothly worked into his fiction in the past (and works in equally smoothly here). At times, this intriguing narrative opens up like a trapdoor to allow a glimpse of deeper levels of meaning and of larger forces at work. The reader can either try to puzzle out what the authors are trying to say, or merely sit back and enjoy the richly textured milieu and engrossing story of one of the best science fiction novels of the year.