Brian Aldiss - Non Stop

From the legendary science fiction Masterworks series.

Curiosity was discouraged in the Greene tribe. Its members lived out their lives in cramped Quarters, hacking away at the encroaching ponics. As to where they were - that was forgotten.
Roy Complain decides to find out. With the renegade priest Marapper, he moves into unmapped territory, where they make a series of discoveries which turn their universe upside-down...
Non-Stop is the classic SF novel of discovery and exploration; a brilliant evocation of a familiar setting seen through the eyes of a primitive.