The sandal username on GitHub sounded familiar, so I checked and realised its the same sandal that does the Prawn PDF library:
<a href="http://github.com/sandal/prawn" rel="nofollow">http://github.com/sandal/prawn</a><p>That gives the book some extra credibility in my eyes.<p>EDIT: OK, now that I've read the foreward, I see Matz himself mentions Gregory's work on Prawn. Doh.