Before you read that book, quitting smoking is perceived as a loss of something that made you feel good. The permanence of this loss -- the anticipation of living the rest of your life without this source of comfort -- leads to cravings.
What the Carr book does is install a new set of associations: smoking is disgusting and your life is going to be so much better once you free yourself from this completely unnecessary weight that's been dragging you down. Instead of viewing quitting as the loss of something good, you learn to view it as the act of freeing yourself from something bad. Now, when you think about a future without smoking, the anticipation is positive: you get to live the rest of your life without this terrible thing intefering with it! And all you have to do is just stop doing it! Sure, you might have a little bit of physical withdrawal for a little while, but isn't that worth it to be permanently free of this awful thing that's been ruining your life?
That's the basic gist, as far as I can tell. In a way, it's just about being optimistic about a life without cigarettes instead of pessimistic.