The Ruby Programming Language covers Ruby 1.8 and 1.9

This book is an updated and expanded version of Ruby in a Nutshell (O’Reilly) by Yukihiro Matsumoto, who is better known as Matz. It is loosely modeled after the classic The C Programming Language (Prentice Hall) by Brian Kernighan and Dennis Ritchie, and aims to document the Ruby language comprehensively 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 take their understanding and mastery of the language to the next level.