A Canticle for Leibowitz
A little humor, a little adventure, some church politics, a lot of warning - Canticle goes a long way without bogging down. The life of a Catholic monk isn't something I know much about but wanting to preserve what knowledge can be held onto in times of darkness is always worth respecting, and I enjoyed the interplay this led to regarding spiritual and secular truths.
I found it interesting to start at, basically, the beginning of the rebirth of human culture and work our way through to the end of an age, all through the eyes of one small group. The people changed, but the larger identity did not. And how about The Wanderer? The same man, a mystic figure, or one in a line of men carrying the same traditional burdens?
Miller did a good job, I think, of presenting the followers of a scientific God. Thon Taddeo expects the monks to be horrified or offended by certain facts, but it's the brothers who have topped him (both in their practical applications, creating the generator and lightbulb, and in certain individuals' ideas about evolution).
Of course, they are still Catholics, preaching that survivors give their pain to Jesus and turn away from the sin of suicide even when faced with miserable, hideous death from radiation poisoning. Frankly, I'm with the doctors on that one - there is nothing to be gained by forcing someone to suffer needlessly.
final thought: The Catholic Church has been such a force in this world, both for good and for violent evil. That they would remain so in a post-nuclear future strikes me as very likely.