I have no problem giving this book a solid +. Not surprising, really, as it must be the fourth or fifth time I've reread it since my teens, each time finding more depth and something to think about. This time around it was Miller's theory of history (or at least the theory of history used to construct the book), which bears some pondering.
Canticle is the story of humanity and the Catholic Church after a near-Armageddon nuclear war wipes out most of society (the book was first published in 1959). Specifically, it follows some of the doings of the Order of St. Leibowitz over several millenia as society struggles back up from barbarism to something just a bit beyond our own technology (slow interstellar travel).
It helps to be acquainted with Roman Catholicism, or at least with Latin, to get through this book, though you can fake it and still get the flavor. There's a good gloss available on the web at http://www.wsu.edu/~brians/science_fiction/canticle.html if you need help with either the language or the religious concepts.
Though packaged as a novel, this is really three separate novellas, all set in the same world, but considerably separated in time. "Fiat Homo" takes place when the war is only a few generations past. "Fiat Lux" takes place a millenium or more later, when science is finally beginning to spring back from fear and superstition. "Fiat Voluntas Tua" takes place another millenium or so in the future, when nuclear weapons are once again possible.
Warning: Spoilers ahead. Click here to read the rest of the review.