Think of it as a Cliff Notes version of the pick axe “Programming Ruby”
Developers of “Sapphiresteel” (the Visual Studios IDE extension for Ruby) had a similar concept, in a pdf book, free to download. http://www.sapphiresteel.com/IMG/pdf/LittleBookOfRuby.pdf
But I believe it is still based on Ruby 1.8… So there is an opportunity here for a 1.9.2 version (which just recently went production).
This would allow for a concise write up of ruby, shorter then “Programming Ruby” but more involved then the Ruby chapter from “Learn seven languages in seven weeks.” My recommendation would be that since sheer volume is being sacrificed for brevity, that perhaps a “Make sure you learn what you can learn” approach is taken. In my opinion the best way to do this, is to present a project at the very beginning of the book and at the end of each chapter, work parts of that project, by either adding or modify the project, based on the concepts learned from a particular chapter. But keep with the project through out the book with the finished project on the last chapter being a professionally looking, complete application.
The same idea can be applied to Rails 3.