There are many books on Ruby and Rails. In my opinion, the following are among the most useful.

Programming Ruby: The Pragmatic Programmer’s Guide

by Dave Thomas, with Chad Fowler and Andy Hunt ($49.95)

ISBN: 978-0-9745-1405-5 (second edition)

ISBN: 978-1-9343-5608-1 (third edition)

Pragmatic: http://www.pragmaticprogrammer.com/titles/ruby/index.html

A vast guide to the Ruby language and libraries, the so-called Pickaxe book is generally considered to be an essential Ruby reference. It’s not a light read, though, and not (in my view) the best first book on Ruby. All the same, you may need it sooner or later. The second edition covers Ruby 1.8; the third edition covers Ruby 1.9.

Beginning Ruby: From Novice to Professional

by Peter Cooper ($39.99)

ISBN: 978-1-5905-9766-8 (first edition)

ISBN: 978-1-4302-2363-4 (second edition)

Apress: http://www.apress.com/

The book provides a gentle introduction to Ruby programming. The explanations are clear, and the code examples are useful. The second edition covers some aspects of Ruby 1.9 but not in much detail. If you already have some programming experience and want an accessible introduction to the world of Ruby, this would be a good book.

The Ruby Way

by Hal Fulton ($39.99)

ISBN: 978-0-6723-2884-8

Addison-Wesley: http://www.awprofessional.com/ruby/

This is a solid, in-depth book on aspects of Ruby programming. In the introductory section, the author states that because of its relative lack of tutorial material, “You probably won’t learn Ruby from this book.” He describes it more as a “sort of ‘inverted reference.’ Rather than looking up the name of a method or a class, you will look things up by function or purpose.” Personally, I think he underestimates the tutorial value of The Ruby Way. The author does, however, assume you are already reasonably adept at programming.

The Well-Grounded Rubyist

by David A. Black ($44.99)

ISBN: 978-1-9339-8865-8 (softbound print book, includes free ebook)

Manning: http://www.manning.com/black2/

This is, in large part, an adaptation of David Black’s previous book, Ruby for Rails, though this time the author concentrates on the Ruby language rather than the Rails framework. It covers Ruby 1.8 and 1.9 but is rather vague on the precise differences between the two versions. It’s a decent introductory book for fairly experienced programmers.

Agile Web Development with Rails

by Sam Ruby, Dave Thomas, and David Heinemeier Hansson ($43.95)

ISBN: 978-1-93435-616-6 (third edition)

ISBN: 978-1-93435-654-8 (fourth edition)

Pragmatic: http://pragprog.com/titles/rails4/agile-web-development-with-rails/

This is the “must-have” book on Rails. Several Ruby programming books might compete for the claim to being essential, but I know of no other Rails book that comes anywhere close to rivaling Agile Web Development with Rails for its comprehensive coverage of its subject. The third edition covers Rails 2. The fourth edition covers Rails 3.