From the book's author's website:
Secret Goal (i.e. why I wrote this book and am giving it away for free online)
I want this book to help bring more people into the Linux kernel development fold. The act of building a customized kernel for your machine is one of the basic tasks needed to become a Linux kernel developer. The more people that try this out, and realize that there is not any real magic behind the whole Linux kernel process, the more people will be willing to jump in and help out in making the kernel the best that it can be.
This book is available under the terms of the Creative Commons Attribution-ShareAlike 2.5 license. That means that you are free to download and redistribute it. The development of the book was made possible, however, by those who purchase a copy from O'Reilly or elsewhere.