Trailblazer introduces several new abstraction layers into Rails. It gives developers structure and architectural guidance and finally answers the question of "Where do I put this kind of code?" in Rails. This book walks you through a realistic development of a Rails application with Trailblazer and discusses every bloody aspect of it.
Still in beta so some things are no longer correct. Also, be careful with code examples, seems like most of them are still in pseudocode state. Book shows promise anyway, waiting for the rest of it.