Originally planned as a second edition to Ruby classic, Ruby In A Nutshell, The Ruby Programming Language is a new book by David Flanagan and Yukihiro Matsumoto (a.k.a. Matz - creator of Ruby) and published by O'Reilly. The book covers both Ruby 1.8 and 1.9 and with its esteemed authors and technical approach, is sure to become a new "Bible" for Ruby developers.
As of the start of 2008 this book is REALLY fresh and up to date. Its style is very direct and matter-of-fact; well suited for existing Ruby developers and proficient developers coming from other languages. The examples are clear and logical and the explanations concise; this is a well edited and authoritative book.
The structure of the book is a delight with ten well-defined chapters (with titles such as Reflection and Metaprogramming, Statements and Control Structures, and Expressions and Operators) that each contain a tree of sections. Consider Chapter 4, Expressions and Operators. A sample dive down to section 220.127.116.11 takes us through 4.5, Assignments; 4.5.5, Parallel Assignment; and finally to 18.104.22.168, One lvalue, multiple rvalues. This is a breath of fresh air in a Ruby reference work.
The only downside, in terms of the thousands who might be browsing Amazon looking for a single Ruby book to start off with, is that this book is so well focused on documenting the core elements of the Ruby language, it doesn't work either as a tutorial / beginner's introduction to Ruby, or as an exhaustive reference work (as, on both fronts, the Pickaxe attempts to be.) This lack of dilution may be an ultimate strength, however, since anyone above the station of "beginner" will be able to learn Ruby thoroughly from this book, use it as a general reference, and then be able to use the exhaustive documentation that comes with Ruby itself to cover the standard library and built-in classes.
In conclusion, whether you're an existing developer or a newcomer to Ruby, you need just three things to be up and running with Ruby in the book / documentation department. Buy this, the Ruby Way (by Hal Fulton), and learn how to use the documentation that comes with Ruby.
This book will act as the "Bible" for Ruby, the Ruby Way will make you an expert, and learning how to use the documentation that comes with Ruby will mean you're not using information that's out of date within a couple of years. The perfect combo! It'll last you for years.