If you dig my blog, you should get this book:
It bills itself as a general introduction, unfortunately that's a marketing mistake. The real subject matter is an introduction to Ruby from a functional programming perspective. This is a huge bonus. There are many books about Ruby that emphasize its OO features, but too few with a clear, succinct, powerful approach to functional programming. You'll likely skip the first few chapters, but still, this is one book you should definitely buy.
Thanks to Clinton Nixon for telling me about it at OSCON!