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 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.
by Huw Collingbourne - No Starch Press
A comprehensive tutorial to the Ruby language. Each chapter is accompanied by ready-to-run source code for all the examples. There is also an Introduction which explains how to use the source code in Ruby In Steel or any other IDE of your choice.