To be honest I have read these books and I have done the koans after having worked with Ruby for a long period. Indeed I discovered that these 3 books (plus the classic Getting Started with Rails in the official documentation) are more than enough to get anyone ready to work and productive since the beginning. More interesting resources I use to look whenever I want to get an insight of a gem or a library are: Railscasts (although a bit outdated), the Rails official documentation and the ruby docs.

RUBY KOANS

Dominate a language is an essential skill for any developer. Frameworks come and go, paradigm shifts, but languages, even if they evolve, will last for long. Variables assignation, arithmetics and math operators, strings, data structures, flow control instructions are the day to day tools of any developer; Ruby Koans is a funny, alternative, TDD based, approach to sharp them.

85
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50
Applicability
85
Usefulness
RubyKoans

PRACTICAL OBJECT ORIENTED DESIGN IN RUBY

Sandi Metz

Thinking in a OO way could be an hard work for a freshman. Good practices have a reason behind them, but is not so easy to spot it at first sight; specially for newbies. Inheritance, mixing, ducktyping, dependency injection, SOLID… A jungle of fancy terms where is very easy to get lost without a well grounded knowledge.
Sandy Metz, with her unique flowing style, helps the reader to build a robust, reasoned background where everything takes its place. This book is a journey into OOP common sense, where the author guides the reader to discover each concept with an easy problem-solution approach. A must read for any ruby developer who wants to start his learning path with the right foot.

100
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100
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100
Usefulness

THE WELL-GROUNDED RUBYIST

David A. Black, Joseph Leo III

This book is not for beginners, it goes deep into ruby language exploring all those technical details that make it so expressive, clear and easy to learn. A large, almost complete, exploration of the language and its features

70
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50
Applicability
90
Usefulness
TheWellGroundedRubyst