Less is more: advantages of compact programming languages
The implementation of Ruby and the standard Ruby libraries is compact, basically divided into the C implementation of Ruby itself and the Ruby code for the standard libraries. If you are going to be working much with Ruby then consider creating at least a project (using Eclipse, IntelliJ, TextMate, etc.) with the library source code. Good for library reference, looking up APIs, and for just reading code.
I would make the same suggestion to developers who use Squeak Smalltalk (or other implementations): you have the source code to the standard libraries: read it!
I also enjoy and am very productive using the Java and Common Lisp frameworks, but these languages with their standard libraries are so huge that I would personally never attempt to dive in and understand their implementations. Not having a mental road map of the implementations of Common Lisp and Java reduces the effectiveness of these platforms.