Ruby Reference
The complete text of the first edition of Programming Ruby, by Thomas and Hunt, is packaged with the One-Click Ruby Installer.
After installing Ruby, select Start → Ruby-185-24 → Ruby Documentation → RubyBook Help
There is a lot worth browsing here. Make sure you take a look at the section titled “Built-in Classes and Methods.”
Background Information
This book is often refered to as the Pickaxe Book. If you continue to use Ruby, consider getting a copy of the second edition.