FORWARD THE FOUNDATION By Isaac Asimov (Doubleday, $255) IT'S appropriate that Isaac Asimov's final novel, finished just before his death last year, should complete the saga that began with Foundation in 1951. The work appeared a year after his first-published novel Pebble in the Sky - which still stands up as an inventive and well-paced work of science fiction - and it is the Foundation series which caps Asimov's contribution to the genre. Written over a period of more than 40 years, the series opens with the Galactic Empire already in tatters after a rapid decline. The mysterious scientist Hari Seldon makes appearances in holographic form during a series of crises he predicted would befall a band of survivors. Psychohistory, the science he created which helps predict the future, is introduced as a key element in the background but not until Forward the Foundation and the final decades of Seldon's life does Asimov go into detail about it. Gone in this work is most of the gee-wizardry of early science fiction, although an element does remain in keeping with the identity of the genre. Instead Forward the Foundation stands as a well-told and exciting tale. There is mystery and intrigue in the imperial court of Cleon I. Various groups lobby for power on the grossly overcrowded home planet and Seldon fights to protect his scientific research under the patronage of the First Minister Eto Demerzel. Emperor Cleon believes that he will ultimately be able to use Seldon's science to halt the decline of the empire and maintain its power through millions of worlds. But Seldon is unsure whether itreally is a science or just the fanciful dream of a fanciful youth. The empire disintegrates at the rate of Seldon's later discoveries and he is accused of accelerating its downfall because of his ability to predict. Whether Asimov, author of over 470 books, meant Seldon's life and creativity to reflect his own and serve as an autobiographical ''What if?'', or whether it is just a footnote to his prodigious literary and scientific output should continue to be a debate for some time.