I've seen the this kernel documentation project emerging a while ago.
It's truly amazing how open source can be used to write a book.
Awsome work!
Unrelated to the post, but another illustration of a great open source book effort is the Stacks Project on algebraic geometry and algebraic stacks:
http://stacks.math.columbia.edu/.
What I've loved about following along on this book is that all I need to do to get a new chapter is 'git pull' .. its truly the future of publishing. ;)