Also check out his follow-up post.

You can purchase Achieving Zen With Auto Layout today and get access to a PDF of the current state of the book, as well as a private GitHub repository where I am writing and storing all the sample code as I go. So far this has proven to be a wonderful way to write, because buyers of the book are able to provide direct feedback to me about the book so I can iron out any confusion or missing things I may not have thought of during my first draft.

I had a chance to talk with Justin about this approach the other day. I love it. I expect this type of interactive approach to creating technical books become more and more popular.