Without a doubt, start with this book (you can get the pdf, or paper, or both):
http://pragmaticprogrammer.com/titles/rails2/index.html
I also found this book really good:
Jeff softiesonrails.com
Yes, I agree with Jeff. The bible absoultly is AWDwR, 2n Ed. [1] But if you want to know what goes under the hood, try to learn Ruby completely.
Ruby for Rails [2] is a *great* book, and Ruby Programming [3] is a must read.
[1] http://pragmaticprogrammer.com/titles/rails2/index.html [2] Ruby for Rails [3] https://pragprog.com/titles/ruby/programming-ruby-2nd-edition/
