I always had a strange perception towards writing tests in Ruby. Ruby is a language where making a change is so easy that you feel empowered to do more than you planned. And it forgives you a lot, allowing you to increase the amount of tech debt that you app accumulates during the development. And sometimes you realize that you can't progress anymore without sacrificing on the stability of your application.
And at this phase usually tests come into play to help you to gain control over your codebase and make it robust.
This book is a great introduction to start writing tests in Ruby. It teaches you concepts that are common, with very little emphasis on RSpec features. In my opinion the coverage of Rspec features is shallow, after reading the book I had to refer to the documentation a lot. Mocking frameworks are covered very poorly.
But as an introductory, especially if Ruby on Rails is your bread and butter this book is great as it's nicely written and doesn't get you bored which is very rare amongst the books about testing.
My score is 3/5