The 'pickaxe,' oft-cited as the definitive guide for aspiring Ruby hackers. I'd have to agree. The edition I have was updated for the current release of Ruby and contains some gems about the inner-workings of the language that I found fascinating. Read it if you are a dyed-in-the-wool Rubyist or even a dispassionate student of computer languages. You'll no doubt find something worthwhile here.
This book was the beginning of a great journey through the ruby language for me which changed my style of programming quite significantly. The writing style is clear and easy to follow whilst at the same time managing to convey a lot of information in a concise way. I wish all programming books were written this way.
Actually I'm reading a downloaded PDF of the third edition that covers Ruby 1.9. This is my first exposure to this language; I like it. I'm happy to say goodbye to PHP (fuck that language, it is made of garbage).
Um...right, about the book: I like it, seems pretty clear and goes through the language using several different strategies. Seems to function well as both a beginner's guide, in depth tutorial, and reference: a rare feat. Even the Perl "camel book" (3rd edition in particular) doesn't really nail that, although it tries.
I was disappointed with the so-called Pickaxe Bible. If you're looking for purely a reference book with some decent explanations, this book is great. It seems pretty exhaustive for beginner-advanced applications of Ruby and takes care to remain somewhat "framework" neutral by always listing both popular frameworks as well as alternatives. However, a few things stopped this from being a great book: lack of applicable examples and inconsistent formatting. The examples were usually very contrived, it would have been great to spend more time giving examples of situations where you may want to use a certain language feature and give an example the reader can work along with. The code examples were also inconsistently formatted. Sometimes you'd have a 3-class example to clearly explain functionality, other times it would be a string of code snippets that felt like with a bit more effort they could have been woven into a more coherent example.
A tutorial this book is not, but as a reference for those who have programmed before and want to answer the question "how do I do XXX in Ruby", this book fits the bill.
The key reference for understanding the Ruby programming language. If you want to be up-to-speed on Ruby you've got to have this book. I also have the PDF version which is great to have if you're not by your bookshelf.