For sixty more days (actually 59 days and 17 hours as of Time.now, or if you prefer to speak Javaish rather than Rubyish, new java.util.Date()) you will be able to download free PDF version of the book by Patrick Lenz Build Your Own Ruby on Rails Web Applications - courtesy of nice guys of Sitepoint.
The book download page is here, no strings attached - all it will cost you is disclosing an email address (book download link will be emailed to you) and bandwidth to get the 20 MB PDF. Mine is downloading right now, so I cannot tell anything about the book yet. It has 447 pages and from the table of content it looks like solid beginner's book.
As a nice surprise it covers Ruby installation under OS-X 10.5 (Leopard) :-). From the rumors I have heard, Leopard should come with Rails preinstalled and with current version of Ruby (1.8.6) - which will not stay current long because Ruby 1.9 as well as Rails 2.0 are quite close.
So hurry up and get your copy until Sitepoint's ISP does not disconnect them from excessive bandwidth usage :-). I am not joking, the first surge of download already did take down their server once :-).
After you download - spread the word.