A Canticle for Liebowitz, by Walter Miller. There's a good Wikipedia page here if you want to discover a bit more about it.
I loved this. Hadn't read any sf for ages - must be at least six months - so it was great to plug back in. Lots of stimulating images, and, of course, it fitted so nicely with my 'After Virtue' interests. Lots of things will stay with me, but perhaps the final debate about euthanasia most of all. Very challenging, but I was grateful that the religious perspective wasn't implicitly ridiculed, in the way that much sf would do.