With only 160 pages you'll be able to cope with that book faster than you could expect, but you should not believe that this book is not interesting. Many points raised are fairly pertinents and Gilligham goes straight to the point. There are no unbacked assertions although there are some points that could have deserved some more attention, like Thomas Beckett or Basile (the knight responsible for Richard I's death). But in the optic of the book, which is to strictly describe the political structure and important political evolutions, they can be regarded as dispensable.
It's simple, everything is referenced and that's highly appreciable, you can read that book and be sure you're not reading a "tabloid version" of the history.
I wouldn't put it, alone, as an absolute reference but you can then expand your reading to more specific details. The end of the book as a rather interesting list on more specific matters which people really interested in the matter should definately check.
Also some events before the "Angevin Empire" like the Norman Conquest could have deserved some more attention but then again it is not the purpose of this book.
Conclusion: If you buy this book you will read only, and strictly only, on the political structures and evolutions of the Angevin Empire. Only on political events that had direct consequences on its geopolitical status and the rest will be just mentioned. But then about the matter it definately doesn't wander around about other issues and maybe not so relevant points (yet not uninteresting).
5 stars for me, mentioning the English used in the book is fairly accessible even to foreign readers (like me).