Andrew Binstock says:

Ever-dependable O'Reilly just released Ruby Programming Language, which is without a doubt the definitive Ruby reference. Not only is it co-authored by Yukihiro "Matz" Matusmoto, the inventor of Ruby, but it is superbly well edited, so that every page is full of useful information presented clearly. And at more than 400 pages, that's a lot of information. Couple this book with The Ruby Cookbook, which I reviewed on this blog, and you have probably the best 1-2 combination for learning and using Ruby.

I haven't seen the book myself, but I trust Andrew's judgment. I certainly agree on the value of The Ruby Cookbook.