A Canticle for Leibowitz by Walter M. Miller Jr
A masterpiece in three parts. The setting is post apocalyptic earth after a mutually assured nuclear destruction scenario occurred in the 20th century.
While most of humanity has undergone the simplification, a small order of Christian monks is dedicated to preserving human civilization. Their patron saint, St. Leibowitz, an engineer from before The Deluge. After a bomb shelter with his blueprint is discovered by a primitive monk, a religious scandal unfolds, reminiscent of what might have happened in the dark ages to Galileo.
Many centuries later the blueprint is finally understood by a scientist frustrated by a lack of truly novel discoveries. And an engineer from the same order builds it. It is a simple electrical generator which powersq a lamp.
Eventually humanity returns to its pre Deluge level of technology and surpasses it, achieving the ability to travel to exoplanets. But political strife is once again at an alarming level. And as earth nears nuclear annihilation again, the order of leibowitz prepares to preserve the blueprints and other critical documents and send them, along with a small order of monks, to another planet for resettlement.
The characters are vividly painted, and the historical projection of the dark ages onto a modern technological background is fascinating and masterfully crafted. The book serves as a stark reminder that there is no natural progression toward better times, and just how fragile our current paradise is.
The threat of nuclear war may feel like a bygone but I'm quite concerned about it still.