It is a quite short book (only 4 chapters) that presents you some more advanced topics of Gradle. Tim Berglund cover here topics such as: file copying & processing tasks, building custom plugins, using hooks to life-cycle events and management of dependencies.
I enjoyed the book. It is easy to read as the authors show many snippets of code as an example for the topics he covers. And because of the relatively short length of the book, you don't need to spend a lot of time reading about details that you probably don't care about.
The book is definitely not for those who are new to Gradle build system. I believe that you should at least be familiar with topics covered in previously published "Building and Testing with Gradle". The authors assumed that the reader already know how to use Gradle and quickly started with describing another features of it.
On the other hand, if you already know how to use it then you might not need to read this book at all. If in your job you need to use more advanced gradle tools, you might as well use only official online documentation, which most probably already covers all the topics from the book. The only benefit you will have from reading Tim's book is that he covered some example step-by-step instructions for using these advanced features, like creating custom plugins.