If you want to get started, my 'Learn Ruby on Rails' [1] covers Rails 4.0. It's for absolute beginners and good preparation for more advanced books. If you're more experienced, start with books by Hartl [2], Fernandez [3], Ruby [4], or Bigg [5].

[1] http://learn-rails.com/learn-ruby-on-rails.html

[2] http://ruby.railstutorial.org/ruby-on-rails-tutorial-book

[3] https://leanpub.com/tr4w

[4] http://pragprog.com/book/rails4/agile-web-development-with-r...

[5] http://www.manning.com/bigg2/