This book is written with the intermediate Ruby developer in mind. You’re already comfortable with programming in Ruby. For example, you may have written some Rails or Sinatra applications, or even dabbled in some metaprogramming, but you’re not an expert in the language.
This book will not cover the basics of Ruby, except when it relates to the subject matter. There are many excellent books and resources that will do a much better job of covering how to program in Ruby. If you are just starting out, or are a little rusty, you should start with The Ruby Way [Ful06], Programming Ruby (2nd edition) [FH04], or The Well-Grounded Rubyist, Second Edition [Bla14] before reading this book.