Very Ruby on Rails specific, but valuable nonetheless.
https://github.com/bf4/learning
http://railsapps.github.com/best-recommended-learning-rails-books-resources.html
http://everydayrails.com/2010/08/04/more-free-ruby-rails-books.html
http://playbook.thoughtbot.com/
http://stackoverflow.com/questions/194812/list-of-freely-available-programming-books
https://github.com/bf4/learning
http://railsapps.github.com/best-recommended-learning-rails-books-resources.html
http://everydayrails.com/2010/08/04/more-free-ruby-rails-books.html
http://playbook.thoughtbot.com/
http://stackoverflow.com/questions/194812/list-of-freely-available-programming-books