Book Level Pages Score(/5)
Design Patterns in Ruby bambi 384 4.5
Eloquent Ruby beginner/intermediate 442 5
Refactoring: Ruby Edition intermediate/professional code reviewer 481 3
Ruby Under a Microscope sensei 362 5
Ruby Best Practices intermediate 330 5
Cucumber Book beginner/nobody 328 3
RSpec Book intermediate 426 5

Design Patterns in Ruby

An example of basic OO patterns. Well written for a beginner Rubyist with great analogy in the OO world. Very shallow explanations of meta/core Ruby internals, but great examples of clean code. Bonus—a chapter dedicated to DSL, with an example.

Eloquent Ruby

This is much closer to Ruby than Design Patterns in Ruby. More focused on practical issues and how to attend to their needs in the Ruby way. Bonus DSL...

Refactoring: Ruby Edition

This one requires a ton of patience. The book explains how to apply various refactoring techniques and (almost) never cross to the red area in the Red-Green-Refactor cycle. It was really hard for me to concentrate on each technique shown because they were as useful as they were boring.

Ruby Under a Microscope

This is one of the best books ever written. The level of detail is, well, microscopic. You cannot get to a much lower level than this, and, at the same time, every higher-level behavior is explained in respect to internal implementation. This book will not leave you guessing how things work, it explains and then demonstrates them with C code implementation.

Ruby Best Practices

This is a self-improvement book for every Rubyist with a couple of years of experience. It is filled with code examples of idioms and methodologies which you've already seen in practice, but this book reveals why they are so dominant over alternatives. The author was the owner/maintainer of some XML builder/parser, so this book provides knowledge that solves real-world problems. A chapter dedicated to gem/project maintenance is a bonus. Bonus^2—there are a lot of references to context-relevant thingies, sometimes an article, other times a book, not necessarily Ruby related but still great reads.

Cucumber Book

Easy to read, but hard to find projects which need that kind of knowledge.

RSpec Book

The community accepted this one as a textbook for writing tests in Ruby, and for good reason. It's everything one needs to start practicing TDD.