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 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 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.
This book is a great collection of ideas, tricks, and skills that could be useful for Hackers. It's a unique extraction reference, summarizes a lot of research and experience. Rubyfu is a book to use not only to read, it's where ruby goes evil.
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.