Free Software Magazine have had their hands on Moving to Ubuntu Linux by Marcel Gagné and gives it a rather positive review.
It's on their site, right here
You should buy this book because it is written by an author who has a lot of experience showing users how to learn to use GNU/Linux, and specifically with this book, Ubuntu. You should also buy this book because it is written very nicely, will keep you entertained, and will show you how to do more with GNU/Linux.
You should not buy this book if you already have several years of experience on GNU/Linux and already have mastered the applications listed above.