This book teaches you not about what functional programming is. The target of this book is to help the reader to learn to think differently. Learning to program with a functional style will require you to do some rewiring in the brain. 

Instead this book helps you to un-learn some of the patterns you've been so very much used to, and learn new functional programming based patterns that help you build consise and efficient solutions to everyday programming problems. At the same time, it will put things in perspective and put functional programming in context, and discuss the limitations of functional programming in general and in Ruby specificially.

The end goal is that this book should help the reader to become a better programmer. A better programmer for me is someone who's flexible in approaching the problem domain. I'm a strong believer that in the near future, it's going to be more and more key for a developer to know many programming languages and many programming styles. 

That's also one of the reasons why I think Ruby is a good programming language to learn about functional programming style. It's far from perfect. It's not at all pure. It has limitations. But it allows you to mix and match different programming styles.