ASFAIK there isn’t a book on ruby-processing, JRubyArt, or propane perhaps (I thinking about writing one). There’s plenty of books on processing, and some where code has been ported to ruby. If you are new to ruby there’s quite a lot available (including on line), but if you are coming from java or processing you can learn a lot just running the examples and reading the code (Rails is a terrible place to start to learn ruby).
If you’re just learning to code for the first time, learning processing with ruby includes most of the examples from Daniel Shiffman’s Learning Processing, an introductory textbook (move
settings to port to JRubyArt). Or there is the more up to date The Nature of Code (more advanced topics) also by Daniel Shiffman, and this case majority of the examples have been ported to ruby. Even more processing books are listed at processing.org. If you want get serious with ruby there’s Sandi Metz’s Practical Object-Oriented Design which should help guide you to write decent ruby code. If you fancy doing a bit of re-factoring get Sandis latest book because there are plenty of opportunities to re-factor both ruby-processing project code, and the example sketches.
Note: Nature of Code also available in Japanese, it would cool to annotate some of the ruby examples?