Let me start out with some praise up front. This is a great little book, and a good investment for learning more about the Ruby idioms that undergird Rails. Jacob's writing is clear and concise, his examples are well chosen and well explained. Buying this book in $10 well spent.
Jacob starts out with a quick run at the Philosophy of Ruby, and the nuts and bolts mechanics of the language. This section takes up about the first quarter of the book. He follows this up with solid explanations of Duck Typing and Symbols. From there he moves on to a section on Blocks and another on Metaprogramming. Finally, he moves on to discussion Domain-Specific Languages.
At each step along the way, Jacob starts with simple examples to cement the readers understanding before moving on to show Rails code, or code in Rails that exhibits the idiom he's explaining. It's a pattern that works quite well.
This shortcut actually started life as a presentation Jacob made to the NYC.rb — a heritage that still shows through in the slide-like landscape pages of the book and Jacob's laid-back writing style.
Given my positive experience with this title, I'm looking forward to reading more of Addison-Wesley's Ruby Shortcuts.