I really like the way he is doing the source code revisions. It's always a pain to be in a book and have broken code and not know why. You can take a sneak peak

I really like the way he is doing the source code revisions. It's always a pain to be in a book and have broken code and not know why. You can take a sneak peak ahead (hg show HEAD+1), or if you make changes and want to revert them, it's really simple as well. Plus getting people to use VCS in any form is a major win in my book.