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 Gregory T. Brown - O'Reilly Media
Ruby Best Practices aims to help Ruby developers from a wide range of skill levels improve their fundamental understanding of the language via exposure to the common practices and idioms that many seasoned Rubyists take for granted.
Ruby is an interpreted, object-oriented scripting language. This book will show how to install and get started with Ruby in your environment. It demonstrates the main features of the language syntax. The text is designed to be self contained.
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.