The Long Tomorrow is one of the first post World War II novels depicting a possible aftermath of nuclear war. It is set only 80 years after the war rather than multiple centuries like A Canticle for Leibowitz. It also contains a deep religious streak. But it has an entirely different psychological perspective and does not raise the issue of mutations.
Instead there is a backlash against science and what is left of American culture is religiously devoted to keeping science repressed. But there is a persistent rumour of some technologically advanced town somewhere out west that still has technology but no one knows where it is. If people believe you are from there they will kill you.
Two young boys accidentally discover that it must actually exist because they find a radio. So they go off searching for the town.
This is science fiction as a thought experiment. There are thousands of more nuclear weapons today than there were in 1955 when this book was first published. Although there does not seem to be as much overt worry about nuclear war today as in the 50s the problem really has not gone away so this book is still just as relevant though many may not think so.
I prefer it to A Canticle for Leibowitz because it does not have the mystical component and seems much more plausible. What I do not understand is why it is so much less well known and mentioned than Leibowitz. Sometimes I think the literary crowd does not like realism in science fiction.