I came across a story about how the author of the book is giving it away online. The goal obviously is to increase the number of kernel developers.
>From the author: 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. The PDF's & DocBook links are available at the bottom of the Author's site: http://www.kroah.com/lkn/ Happy reading.
