The quality of a book written by the author (inventor) of a programming language the book is descriping can only be expected to be both poor and excellent (though I have experienced cases where the book was excellent) - therefore the average rating good.
Why is it poor?
If you are new to C++ it aint worth it. That's one. Don't buy this book until you have worked with it for a while and know your way around. He is very technical in his descriptions and sometimes lack in depth descriptions of the subject he is addressing. The books isn't good for inquiries. It's difficult to find what you need.
Why is it excellent?
Well, it's written by the man who made the language. He should know everything there is to know about it - and he does. The book has everything you need, and when you have used it for a while, you get better at finding it.
Why should you buy it?
If you are into C++ this is a must-have book. You can't honestly say you are a qualified C++ programmer if you don't have this book on your desk at home (and at work).