Programming Ruby, The Pragmatic Programmer’s Guide, First Edition (free) is one of my favorite Ruby learning books. Paired with Peter Cooper’s Beginning Ruby: From Novice to Professional, it’s a great resource for learning Ruby.
One thing missing from the online version of the book are the some of the figures that show how Ruby’s classes and meta classes work. Luckily I found these images online a few years ago. The image names are named according to their chapters. Enjoy. 🙂