I had one of the authors of this book, Remzi Arpaci-Dusseau, as a professor for my OS class last spring. It was one of the best classes I've ever taken. This book is a direct result of the effort he puts into his teaching. In light of the recent submission about XV6 (http://news.ycombinator.com/item?id=3753216), this book will be very helpful if you want to dig into its internals.