Here's a quick description of what the book covers.
MacRuby In Action is a tutorial for Ruby developers who want to code for OS X without learning Objective C. In it, you'll learn the ins and outs of the MacRuby language, including straightforward examples of creating OS X applications using Cocoa components. The core of the book is an example-driven tour of the Cocoa framework from the Ruby perspective that focuses on getting things done. You'll explore the key Cocoa design patterns, along with a few alternative twists that MacRuby makes possible. Finally, the book takes you on a tour through some high-value techniques including system scripting, automated testing practices, and getting your application ready for the Mac App Store.