THE CITY AND THE STARS by Arthur C. Clarke (Bantam: $4.95). Nearly 40 years after its initial publication, "The City and the Stars" remains one of finest science-fiction novels of the postwar era. Clarke has explored the theme of the inevitable stagnation brought on by an artificial, technologically maintained paradise in other novels and short stories, but never as gracefully. The staggering time scale--the book is set more than one billion years in future--is taken from Olaf Stapledon's "Last and First Men," but Clarke offers a more humane vision of the future that emphasizes mankind's need to explore, to challenge and to be challenged. A well-crafted book by any standard of mass-market fiction, "The City and the Stars" is one of the first installments in Bantam's reissue of Clarke's work.