To really understand Ruby you need to understand its Class/Object model. And to do that, read Brian Marick’s mini-book “A Little Ruby, a Lot of Objects.” It’s modeled after The Little Schemer, but using Ruby of course.
Using the Socratic method, it goes through a series of whimsical questions and answers as it probes deeply into Ruby Class/Object model. Chapter 3, titled “Turtles All the Way Down”, is where it all comes together. By the time you are done you feel like you could have come up with Ruby’s model yourself!
Sadly, Brian never finished or published the book. Written way back in 2001, at this point I assume most Rubyists don’t even know it exists. Since I still like to refer back to it from time-to-time, I’ve put together its three chapters into a single pdf and am hosting a copy.