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 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.
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 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.
Everything from installing Ruby and the basics of the language through to topics such as arrays, file handling and object-oriented programming are covered. Written both for experienced in other programming languages and for novices.