I usually describe TWGR as a "re-purposing" of R4R. It's all Ruby, with some material adapted from R4R (but also completely reviewed, revised, and updated), as well as a lot of new material, and all updated for Ruby 1.9.1.
Here's the story. Manning asked me whether I wanted to do a second edition of R4R. I replied that what I really wanted to do was write a Ruby book. Of course, I wasn't starting from scratch. Still, TWGR was actually harder to write, and took longer, than I'd expected. That was in a sense a blessing in disguise, since it meant that by the time the book was being wrapped up and printed, 1.9.1 had already appeared.
Part of the rationale for doing it this way, instead of just updating R4R in place, was my belief that while the "for Rails" part of R4R was very important, most Rails developers today understand that they are, in fact, Ruby developers, and they are interested generally in the Ruby language. And so are lots of other people. So even though it isn't specifically "for Rails", TWGR is certainly for Rails developers, as well as other Ruby programmers and non-Ruby programmers who are setting out to learn Ruby from scratch.
I hope that helps put it in context.
Ruby training coming up in September! David A. Black and Erik Kastner team up for fast-paced, four-day Ruby intro, in New Jersey, September 14-17. See http://rubyurl.com/vmzN or contact David.