GTM is a really powerful tool and understanding the data layer is the key to getting the most out of Google Tag Manager.
What is so different about this book is that, firstly it is only the book on data layer ever published. Second it actually teaches you the JavaScript required to create a data layer.
All other resources on data layers assume that you already know JavaScript or they simply tell you to figure it out on your own. But you really cannot create, use or even understand data layers, if you do not understand the JavaScript behind it.