A Canticle for Leibowitz by Walter Miller

A classic science fiction/fantasy book from the 1960s. The book is set in three time periods. Current times when a monk finds a fall out shelter from a nuclear holocaust. People attributed scientific knowledge to the holocaust so learning became outlawed.
The second part is set in 3174 A.D. during a new era of enlightenment. A man visits an abbey to study their memorabilia, which has been hidden from others.
The third part is set in 3781 when technology has advanced. Another nuclear war happens and monks are sent into space where they can spread and preserve knowledge.