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Practical Object-Oriented Design in Ruby by Sandi Metz. http://www.poodr.com/

Maybe it's a matter of taste, but I found the GoF book really boring (and a little self hyped TBH).

If you want a good alternative to the GoF book try Head First Design Patterns. I findit much more approachable and much more enjoyable to read. http://www.amazon.com/Head-First-Design-Patterns-Freeman-ebo...