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The Ruby Programming Language by David Flanagan and Yukihiro Matsumoto (the creator of Ruby) is very good.

The coverage, speed and size (half the size of Pickaxe book) make you productive quickly.

...and it's intentionally structured in a way that mirrors K&R!

Programming Ruby (aka "the Pickaxe").

I never fully grokked Ruby's object model until I read that book.

I reread it often and always discover/re-discover useful information.

I've read many Ruby books over 6 years. The best, in my opinion, is David Black's "The Well-Grounded Rubyist". I read it just last year as an experienced Rubyist. I thought it had a fantastic, no-magic take on the language that offered a better perspective on scope, metaprogramming, and general language design than any other book I've read. That said, I haven't read the Matz book.