I've largely been unsatisfied with most Ruby books (especially the
oft-referenced Pickaxe book) because they don't quite go to the depth
that you need day-to-day. Often a Ruby hacker needs to resort to
google-fu to understand certain particularly gnarly parts of the
language.
I flipped through this and it looks like a well written,
well-structured overview of the Ruby language. Thanks to Huw for a
great book and for
providing it
online here.