This is a very specific book, for a very specific audience. If you just need to do some simple auth stuff with AAD, you probably don't need this book. It might be useful for background, but you probably don't need it. If you do need to dig into the guts of this stuff, then you might want to read this.
If you're not sure if this book is for you, read the introduction, specifically the "Who should read this book" section and the "This book might not be for you if..." section. That should help you set expectations.
This book was published in 2016, so it's already out of date in terms of some specifics, but the underlying concepts and low-level stuff is still applicable.
The text is reasonably clear and well-written, given the complexity of the subject matter. Honestly, i had some trouble working my way through some of it though. I don't know if that's just me, or if the text could be a little clearer. Probably a little of both.