WARNING: Some getting-to-know-you content; deal with it.
My father gave me a copy of this for my birthday as a teenager; he told me it had been one of his favorites. Though I've been a bookworm since I learned how to read, my copy remained unread until well after I had graduated from college, until one day I grabbed it at random before catching the train to work.
I started reading it on the long ride from Baltimore to Annapolis and for the next few days got little done--at work or at home--until I had finished. Even afterwards I felt like a clubbed seal and the so-called real world seemed dim in comparison to the world inhabited by the Albertian Order of Leibowitz. It is, in short, a stick of dynamite cleverly disguised to look like a trade paperback.
The theme is folly, hubris, and the temporary nature of things that we consider permanent. If you were to distill the whole book to one proverb, it would be "This too shall pass." But despite the essential pessimism of the plot (Miller seems to believe that a civilization that reaches a high enough level of advancement will inevitably destroy itself eventually) the characters are portrayed with warm sympathy. To top it off, in many places the writing is thigh-slapping hilarious.
This book should be required reading for any politician at the level of city councilman or above. Unfortunately, I have no hope that this will ever be so.