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 Mark Slagell
Ruby is an easy object oriented language: it is designed to be easily read and written. This User's Guide will get you started at invoking and using ruby, and give you insights into ruby's nature that you may not get from the reference manual.
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.
by Avdi Grimm - objectsonrails.com
The book is documenting some guidelines and ideas for applying classic object-oriented thought to Ruby on Rails applications. It is aimed at the working Rails developer who is looking to grow and evolve Rails projects while keeping them flexible.