Programming Ruby: A Pragmatic Programmers Guide by AndrewHunt, ChadFowler, and DavidThomas (ISBN 0974514055)
Also known as the PickAxeBook because of the pickaxe on the cover.
The first edition of the book (for RubyLanguage 1.6) is available online as OpenContent at http://www.rubycentral.com/book/. Also with frames navigation at http://www.ruby-doc.org/docs/ProgrammingRuby/ and http://phrogz.net/programmingruby/.