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The Well-Grounded Rubyist (2nd edition) by David A. Black (Amazon)
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Programming Ruby (2nd edition): The Pragmatic Programmers' Guide by Dave Thomas, with Chad Fowler and Andy Hunt (O'Reilly)
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Agile Web Development with Rails 4 by Sam Ruby (PragProg)
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The RSpec Book: Behaviour-Driven Development with RSpec, Cucumber, and Friends by David Chelimsky, Dave Astels, Zach Dennis, Aslak Hellesøy, Bryan Helmkamp, Dan North (PragProg)
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Practical Object-Oriented Design in Ruby by Sandi Metz (site)
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The Passionate Programmer: Creating a Remarkable Career in Software Development by Chad Fowler (PragProg)
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EXCEPTIONAL RUBY by Avdi Grimm (site)
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Confident Ruby by Avdi Grimm (site)
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The Clean Coder: A Code of Conduct for Professional Programmers (Amazon)
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Metaprogramming Ruby 2: Program Like the Ruby Pros by Paolo Perrotta (PragProg) + Ruby Metaprogramming Screencast by Dave Thomas (PragProg)
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Refactoring: Improving the Design of Existing Code (Amazon)
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Everyday Rails Testing with RSpec (Leanpub)
- Ruby under microscope by Pat Shaughnessy (site)
- Rails as she spoke by Giles Bowkett (site)
- Ruby Hacking Guide (site)
- Growing Rails Applications in Practice (PragProg)
- Crafting Rails 4 Applications (Amazon)
- Working with ruby threads (PragProg)
- Ruby performance optimization (PragProg)
- Ruby Science: The reference for writing fantastic Rails applications (link)