Reading "The Rails 3 Way"
While Rails itself is "opinionated" Obie's book is even more so: he bases his business on Rails and the book reflects the way things are done in his company. The book uses Haml and RSpec exclusively. I still mostly use Test::Unit and Erb, and the book will probably not change that (but it might!).
I found Paolo Perrotta's "Metaprogramming Ruby" to be a great resource for getting more into the low level details of Ruby and I expect that "The Rails 3 Way" will serve the same purpose for Rails.