Ruby Hacking Guide
by Minero Aoki
The RHG is a book that explains how the ruby interpreter (the official C implementation of the Ruby language) works internally. To fully understand it, you need a good knowledge of C and Ruby. This book is going to investigate ruby as a whole.
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by Matt Aimonetti - O'Reilly Media
MacRuby is Apple's implementation of the Ruby programming language which uses Objective-C runtime. This book is an introduction to MacRuby. Topics covered: Introduction, The Cocoa Environment, Foundation, Application Kit, Core Data, Xcode.
by Huw Collingbourne - Dark Neon Ltd.
A straightforward introduction to the fundamentals of Ruby programming: strings, numbers, objects, methods, class hierarchies, arrays, iterators, hashes, modules, mixins and much more. Small, self-contained programming samples are included.
by why the lucky stiff
This is just a small illustrated online-only Ruby book. It won't crush you. It's light as a feather (because I haven't finished it yet--hehe). And there's a reason this book will stay light: because Ruby is simple to learn.
by Huw Collingbourne - No Starch Press
A comprehensive tutorial to the Ruby language. Each chapter is accompanied by ready-to-run source code for all the examples. There is also an Introduction which explains how to use the source code in Ruby In Steel or any other IDE of your choice.