The Jewels of Aptor #2♦ By Samuel R. Delany

Delany's first novel, from 1962, serves as a sort of prologue to the subsequent Captives of the Flame, 1963. Set several centuries after the Great Fire -- a nuclear holocaust -- In a post-atomic future, when civilization has regressed to something near the Middle Ages or even before, a young student and poet, Geo, takes a job as a sailor on a boat. He travels with a strange passenger, a priestess of the goddess Argo, who is heading toward a mysterious land of mutants and high radiation, called Aptor. The journey is presumably made to recapture a young priestess of Argo: her daughter (her little sister in the Ace Double version), who has been kidnapped by the forces of the dark god Hama.. (Summary by BellonaTimes and Wikipedia)