A Little Ruby, A Lot of Objects (Brian Marick)
The goal of this book is to teach you a way to think about computation, to show you how far you can take a simple idea: that all computation consists of sending messages to objects. Prerequisites is reader will know at least one programming language, not necessarily.
Author(s): Brian Marick
NA Pages 
This book is the only Rails book that does test-driven development the entire time, an approach highly recommended by the experts but which has never been so clearly demonstrated before. Finally, by including Git, GitHub, and Heroku in the demo examples, the author really gives you a feel for what it’s like to do a real-world project. The tutorial’s code examples are not in isolation.
Author(s): Michael Hartl
NA Pages
Cookin' with Ruby on Rails Designing for Testability
This tutorial is intended for use with Rails 1.2.6.It has not yet been updated to support Rails 2.x.
Author(s): Bill Walton
NA Pages
