Space opera at its best--wild and fast and furious, as only Robert Silverberg could write it   As a young man, Robert Silverberg was a science fiction prodigy, turning out top-flight stories in the blink of an eye. Though written quickly, Silverberg's early prose already showed evidence of the literary and imaginative qualities that would make him a giant in the field. In "Slaves of the Star Giants," electrician Lloyd Harkins finds himself transported from 1956 into a desolate far-future Earth ruled by monstrous aliens. And in the gripping title story, a spacer named Barsac risks his life and sanity to free a friend from the clutches of an evil cult--by joining the cult himself.   Filled with slam-bang action and dazzling speculation, these seven novellas pay eloquent homage to the Golden Age of science fiction and anticipate the groundbreaking work that has become Silverberg's legacy.