Ruby aficionados have been working for the last few years under a serious handicapt: there was not good, up-to-date reference on their favorite language. Sure, the Pickaxe book provided some guidance, but it's a hybrid work--part tutorial, part reference. And the reference section was a summary, rather than an in-depth exposition.
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.