The most popular book is Allen Carr's Easy way to quit smoking. By the way, if you quit a few days ago, the QuitNow team would recommend that you avoid this book. It says that you can smoke while reading it, but the people we interviewed relapsed. We could say that it is a book to warm up with before actually quitting.
The second most popular book is How to quit smoking without feeling like sh*t. It talks about quitting gradually instead of cold turkey. Remember that the W.H.O. encourages you to quit cold turkey because studies have shown it is easier and has fewer relapses.
The last one we would recommend is the miniature edition of Quit: read this book and quit smoking. It also says that you should quit gradually, which the W.H.O. disagrees with, but it has a good concept: it is the same size as a pack of cigarettes. So you can literally replace your old pack with this book.