JASON AND MEDEIA
by JOHN GARDNER
354 pages. Knopf. $7.95.
Once upon a legendary time there was this young Greek hero−handsome as a god. naturally, and the son of a king−who was promised the throne if he could perform one small, one merely impossible quest: bring back from far-off Asia Minor a golden fleece guarded by a monster that never slept.
Once upon a not-so-legendary time−a time when writers have to borrow their heroes and their morals if not their plots−Jason's story is getting retold by the most unlikely of bards: a professor of Old...