This is generally great advice, but the Ruby standard library isn't exactly the best to read if you want to learn. Don't get me wrong, it works, but the code is generally very, very old. Nobody knew how to write good Ruby code back then.
If it has not been rewritten using modern (good) Ruby style I guess it's still turning a perfect cartwheel. While reading Eloquent Ruby (by Russ Olsen) Ruby's set standard library was recommended for source reading. It was an awesome read! I think standard libraries are really good at demonstrating at least two things: the language's preferred coding style and implementation of data structures and algorithms using the language's primitives. For these 2 reasons, they're still worth reading, imo.
If you're speaking of code reading, I find looking at the source code of Rubinius very interesting.
Ruby has so much utility wrapped up in places like Enumerator, that you don't see a lot of examples of low-level data structure implementation in the wild. You could look at MRI Ruby, you'd be looking at a lot of C, which tells you a lot about how Ruby works, but doesn't show you much actual Ruby. That's where Rubinius is really great. You get to see how a smart team would implement Ruby... in Ruby.
I loved it. Beyond pure technical chops, Russ Olsen is a great writer which makes his books stand out to me from other programming books I've read. In Design Patterns in Ruby each chapter handles a different pattern with a simple example that's easy to understand. And at the end of each chapter Olsen points out where that pattern is used "in the wild" in popular ruby projects if you'd like to see a larger example.
