Orphans of the Sky
Orphans of the Sky (ISBN 0671318454) is a 1951 science fiction novella by Robert A. Heinlein, consisting of two parts: Universe and Common Sense.
In the novella, a gigantic, cylindrical generation ship is cruising pilotless toward a distant solar system, inhabited by a crew that has forgotten their origins over time and lapsed into superstition. Most crew members lead a simple bucolic life of farming, never venturing to the upper decks where the "muties" (mutants or mutineers) dwell. One day, Hugh Hoyland starts exploring and finds the astonishing secret of the ship's purpose.