The Ruby Programming Language is the authoritative guide to Ruby and provides comprehensive coverage of versions 1.8 and 1.9 of the language. It was written (and illustrated!) by an all-star team:
The book also includes a long and thorough introduction to the rich API of the Ruby platform, demonstrating — with heavily-commented example code — Ruby's facilities for text processing, numeric manipulation, collections, input/output, networking, and concurrency. An entire chapter is devoted to Ruby's metaprogramming capabilities.
The Ruby Programming Language documents the Ruby language definitively but without the formality of a language specification. It is written for experienced programmers who are new to Ruby, and for current Ruby programmers who want to challenge their understanding and increase their mastery of the language.