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 Dan Nguyen - bastardsbook.com
The Bastards Book of Ruby is an introduction to programming and its practical uses for journalists, researchers, scientists, analysts, and anyone else whose job is to seek out, make sense from, and show the hard-to-find data.
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 Mitch Guthrie - bigfatrails.com
A book designed around teaching Ruby on Rails in a precept upon precept fashion. Both theory and practice driving your knowledge. If you are a beginner, the purpose of this book is to simply get you there in small, manageable steps.
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.