Design Patterns: Elements of Reusable Object-Oriented Software by Gamma, Johnson, Helm, Vlissides. Addison-Wesley, 1994. ISBN: 0201633612. This is the Gang of Four's book on patterns.
Programming Ruby by David Thomas and Andrew Hunt. Addison-Wesley, 2000. ISBN: 0201710897. This book is a must for all Ruby programmers.
ruby-talk Mailing List: www.ruby-lang.org/en/ml.html
Ruby Garden, a News Portal for Ruby: www.rubygarden.org
Ruby Home Page: www.ruby-lang.org Contains an extensive directory of links to Ruby resources on the Internet. You can also download the Ruby sources from this page.
“Using Mix-ins with Python” by Chuck Esterbrook. In Linux Journal issue 84 (April 2001). An on-line version of the article can be found at www.linuxjournal.com/article/4540.