Just another detail:

The more famous of the two books,
"A Canticle for Leibowitz,"
may not have been conceived as a novel at all.
I would have to go looking for references and all, but if memory serves,

"A Canticle for Leibowitz" has its origins
in a series of short stories that first appeared
in periodicals/magazines,
and were later compiled under the well-known title.

So, Miller was really no novelist at all ...
his expertise was greater in shorter works of fiction
for submission to periodicals, if I understand this right.

If I'm not mistaken in this,
then this fact goes a long way towards
explaining why these two "Leibowitz" books are so very different.