"A Canticle for Leibowitz" by Walter M. Miller Jr. (1960)
"It was said that God, in order to test mankind which had become swelled with pride as in the time of Noah, had commanded the wise men of that age, among them the Blessed Leibowitz, to devise great engines of war such as had never before been upon the Earth, weapons of such might that they contained the very fires of Hell..."
Is this a science fiction novel? I'm not entirely sure. It certainly falls within the domain of "speculative fiction", though I have trouble with that particular term. What kind of fiction isn't speculative?
Science fiction or no, this is a great book. It takes a while to get going, but when it finally does it has a depth that few other books can equal. The plot revolves around a group of monks living in a post-apocalyptic age, and their attempts to preserve ancient knowledge in the face of our collective tendency toward barbarism. As you might imagine, this book sets its sights high, but anyone who perseveres to the end will find himself (or herself) well-rewarded for the effort.
This is the only novel that Miller ever wrote. A victim of post-traumatic stress disorder incurred during his service in World War II, he later struggled with depression and ultimately took his own life in 1996. Both his depression and his service in the war inform "Canticle," and if someone was only going to write one book in their lifetime, this would be the book to write. A sequel of sorts exists, but it was completed by another author.
WARNING: There's a lot of Latin in this one. Those who've studied Latin might want to review a bit before reading this book. Reading the Canterbury Tales beforehand might also help.