If you prefer to read books on your computer screen, here are some good choices—and they are all free.

Learn to Program

The first edition of Chris Pine’s book provides a gentle introduction to Ruby.

http://www.pine.fm/LearnToProgram/

Programming Ruby: The Pragmatic Programmer’s Guide

This is the first edition of the well-known “Pickaxe” book.

http://www.ruby-doc.org/docs/ProgrammingRuby/

The Little Book of Ruby

This is the baby brother to the book you are currently reading.

http://www.sapphiresteel.com/The-Little-Book-Of-Ruby/