An outstanding book and one of my favorite subjects of SciFi, Generations Ships. As well as great cover art.
Originally published as Non-Stop this is on par with, Orphans of the Sky by Heinlein.
Starship, a very well written and dark story. Almost a novella by our standard of modern SciFi books. The Story takes place on a, surprise, Starship and the inhabitants have forgotten where they are for reasons later explained. One of the nice things about this book, most every mystery is explained as the story goes on.
Danger Will Robinson Spoiler alert.
Set in a primitive world of corridors choked full of "Ponics", mutated plants literally choking the corridors of the ship, talking rats who have psychic caged pets and giants who seam to come and go by magic. Primitive tribes fight for scarce resources and control of their world the ship.
Concepts like social devolution borrowed from books like "Lord of the Flies" and really nice details of what happens when a small area losses it artificial gravity as well as humans evolving to adapt to there environment, ah la Fosters Effect, make this a must read for all SciFi Fans.