If you have good links of your own, please add them in the comments! I would imagine many readers of HN are interested to some degree in Rails 3, so hope you find this useful.
Videos Discussing Rails 2 vs Rails 3:
http://rubyonrails.org/screencasts/rails3
Great Rails 3 Guide:
http://railstutorial.org/book (free online, paid PDF)
List of Freely Avaliable Programming Books:
http://stackoverflow.com/questions/194812/list-of-freely-available-programming-books (many books lited, not just Ruby)
Free PDFs for your eBook Reader:
http://stackoverflow.com/questions/172380/programming-texts-and-reference-material-for-my-kindle-creating-the-ultimate-re (again, several flavours available)