In the early 90s, I read all of the Hugo winners at the time. A Canticle for Leibowitz was one of my favorites. A few years later, it was one of the three major influences we used when making the original Fallout (along with Road Warrior and City of Lost Children).
It's strange, you rarely hear it mentioned in the same breath as other post-apocalyptic literature like A Boy and His Dog or I Am Legend . I was only made aware of it while reading an article about the making of Fallout. So, thanks for the indirect recommendation!
The Road, McCarthy
And in a different sort of apocalypse:
Nueromancer, Gibson