Minero AOKI's Ruby Hacking Guide is an excellent introduction to Ruby's internals. It is being translated into English at http://rhg.rubyforge.org/.
Eigenclass (http://eigenclass.org/) has several more technical articles on Ruby.
Evil.rb is a library for accessing the internals of Ruby objects.
It can change objects' internal state, traverse and examine the klass and super pointers, change an object's class, and
cause general mayhem. Use with caution. It is available at http:// rubyforge.org/projects/evil/. Mauricio Fernández
gives a taste of Evil at http://eigenclass. org/hiki.rb?evil.rb+dl+and+unfreeze.
Jamis Buck has a very detailed exploration of the Rails routing code, as well as several other difficult parts of Rails, at http://weblog.jamisbuck.org/under-the-hood.
One of the easiest-to-understand, most well-architectured pieces of Ruby software I have seen is Capistrano 2, also developed by Jamis Buck. Not only does Capistrano have a very clean API, it is extremely well built from the bottom up. If you haven't been under Capistrano's hood, it will be well worth your time. The source is available via Subversion from http://svn.rubyonrails.org/rails/tools/capistrano/.
Mark Jason Dominus's book Higher-Order Perl (Morgan Kaufmann Publishers) was revolutionary in introducing functional programming concepts into Perl. When Higher-Order Perl was released in 2005, Perl was a language not typically known for its functional programming support. Most of the examples in the book can be translated fairly readily into Ruby; this is a good exercise if you are familiar with Perl. James Edward Gray II has written up his version in his "Higher-Order Ruby" series, at http://blog.grayproductions.net/categories/higherorder_ruby.
The Ruby Programming Language, by David Flanagan and Yukihiro Matsumoto (O'Reilly), is a book covering both Ruby 1.8 and 1.9. It is due out in January 2008. The book includes a section on functional programming techniques in Ruby.