A Canticle for Leibowitz, by Walter Miller

Dazzling early (1960) post-apocalyptic Sci-fi, with a heartbreaking sense of inevitability and repetition to our own self-destruction. Obviously born of nuclear nightmares, this still had  a strong message to send, despite a certain datedness. I can't believe I had never read this before, this was definitely filling in a gap for me. I still think On The Beach is a more powerful novel, but this went to darker places after the final clouds rained poison, and suggested that we would do it all over again.