Not life-changing, IMHO.
You can buy it on amazon as a mass market paperback. http://www.amazon.com/Canticle-Leibowitz-Walter-Miller-Jr/dp/0553273817. You can also listen to it online. Warning: there are various scam .pdf's of this floating around torrents and web pages. Do NOT download them. They all contain trojans. If you drive a lot (I do) download the vogg or mp3 audio parts and burn them to a cd and listen to it in the car. https://archive.org/details/ACanticleForLiebowitz
Whoever owns the rights to this book should strive to make it more widely available, the demand seems to be there.
For a long while hubby and I were commuting 2 hours a day and we really enjoyed listening to audio books.
And actually the best, by far, was “World War Z”, it was an abridgement, but they did it like a radio play with all different people reading their part of the story. And Mark Hamill was excellent as the main protagonist. Highly recommended!