Ruby User's Guide
by Mark Slagell
Ruby is "an easy object oriented language". It may seem a little strange at first, but 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.
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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.
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 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.