Why do you write [an entire kernel, an operating system, an RDBMS, a web application framework, ...] and offer it for free? Sometimes, the journey of creation and the sharing are more important than any remuneration you might otherwise get.
I hope he publishes a print version of the finished book, like Miran Lipovača did with Learn You a Haskell for Great Good! If someone releases a free book that has immense value to me, I'm more than willing to support them by buying a wonderful print version. There's something wonderful about having the ergonomics of a print book, combined with the ability to copy and paste and follow links from a PDF...
Yes, I'll signup with service like lulu.com in future, for those who want hardback, when I'll feel I finish the thing, maybe 1-2 years later.