It simplifies a some concepts regarding the inner workings git, however I strongly dislike the structuring of the information within the chapters - the way they're arranged seems unintuitive. As an example, the entirety of chapter 8 can be summarized more aptly by the corresponding page on git-scm. It also sometimes references syntax that it made no prior definition for and other times leaves certain chapter-relevant syntax definitions out for no known reason (which the reader might find useful).
I believe it's useful for getting a general idea of where to start looking for more information, because of its explanations, but I don't see it being useful to beginners or advanced git users.