If I had to pick two..
Design Patterns in Ruby by Russ Olsen is the first technical book in a very long time that I have enjoyed reading from cover to cover.
It's more than just a naïve translation of the classic GoF patterns. Olsen manages the dual trick of not only demonstrating how the classic patterns can still be relevant in Ruby, but how to approach them with the full power of ruby at your disposal.
I liked the way that Olsen avoided doing bare minimum implementations. So when looking at the Composite pattern, he spruces things up with a little operator overloading. And where ruby affords a number of possible approaches, these get discussed and compared (like with the Decorator pattern).
The final chapters in the book present a few additional patterns that go beyond the GoF and are particularly topical and relevant for ruby: DSLs, meta-programming, and convention over configuration.
In short, Design Patterns in Ruby is a grand tour, an effective tutorial in a selection of ruby practices, and ultimately a very enjoyable, rewarding, and sometimes even funny book to read.
The second book I'd stowaway with is Ruby Best Practices by Gregory Brown.
It doesn't pretend to be encyclopedic in the manner of The Ruby Way. However, where sometimes I find The Ruby Way curtails topics just when they start to get interesting, Brown dives deep with Ruby Best Practices.
Clear examples are accompanied by thoughtful and full treatments of the subject at hand. It has particularly useful focus on "Mastering the Dynamic Toolkit", "Text Processing", "Functional Programming Techniques", and "Designing Beautiful APIs".
So they're my picks. Now, obviously these are not ideal books for learning ruby from scratch, but once you're past the basics these are the two at the top of my pile;-)
Anyone willing to counter with their top two picks? Agree or disagree with my choice?
Soundtrack for this post: I Like Your Old Stuff Better than Your New Stuff - Regurgitator from the album Unit Re-Booted