99 Bottles of OOP A Practical Guide to Object-Oriented Design by Sandi Metz

Confident Ruby 32 Patterns for Joyful Coding by Avdi Grimm, Sandi Metz

Eloquent Ruby (Addison-Wesley Professional Ruby Series) by Russ Olsen

O'Reilly - Ruby Pocket Reference

Practical Object-Oriented Design An Agile Primer Using Ruby by Sandi Metz

Programming Ruby 1.9 2.0, 4th Edition The Pragmatic Programmers Guide by Dave Thomas, with Chad Fowler, Andy Hunt

Ruby Under a Microscope An Illustrated Guide to Ruby Internals by Pat Shaughnessy

The Ruby Programming Language (Covers Ruby 1.8 and 1.9) (David Flanagan, Yukihiro Matsumoto)

The Well-Grounded Rubyist by David A. Black, Joseph Leo III