Dated at this point but still a valuable and interesting book. It starts from ground zero and attempt to recreate a successful web framework (Rails) step by step. While doing so it points out all sorts of interesting features of Ruby and how Rails is able to use them to get "magical" behavior.
This is a great concept. But this book really needs a professional editor. The code referenced has errors, particularly towards the end. Still, the concept is so great that you can almost forgive the sloppy execution. Almost.
If you want to understand the Ruby on Rails internals you should pick up this book. In my eyes this is one of the best advanced book on web development and building your own web framework as an exercise.