Walter M. Miller's three-part novel A Canticle for Leibowitz is one of the acknowledged classics of science fiction... that doesn't get talked about nearly enough. Over in the New Yorker, there's a fascinating, must-read essay about this book, putting it in context.
When we try to imagine the world after an apocalypse, we often end up thinking of a particular era from history. It's not surprising — because the end of the world as we know it often means we'll lose a lot of the fruits of progress. People often talk about bombing someone "back to the Stone Age," for example.