I recommend A Canticle for Leibowitz unconditionally. It's rare that a science fiction book with a strong element of religion can keep my attention, but this one did.

Plot synopsis: nuclear war destroys civilization on earth, and the Catholic church manages to preserve some ancient knowledge (textbooks and such) in monasteries. The monks of the Order of St. Leibowitz guard the sacred relics of knowledge (the blessed blueprint, the sacred shopping list and the holy shrine of Fallout Shelter) until such time as the world is again interested in them.

I liked the fact that the author didn't reveal everything or tie up all the loose ends. For example, we never find out for sure who exactly the Old Jew is (although there are a lot of clues, so you should be able to figure it out.) That's not to say the author didn't bring the book to a satisfactory conclusion--he did--but he left just enough up to the imagination of the reader.