So as I noted in another comment, I read this several times when I was very young, so I reserved a copy from my library and picked it up yesterday.

This is a newer copy than what I had. For one thing, it was published in 1959 (though the copyright is 1939) and it mentions “author of Saint Leibowitz and the Wild Horse Woman” on the cover, which what I read way back then definitely did not.

And, I think what I read had a red cover (paperback)

I assume this is the revised version you mentioned because I do not remember the third part Fiat Voluntas Tua.

Update: now that I;m reading that part again, I do remember it,