Free Book: Operating Systems: Four Easy Pieces

I had Remzi, one of the authors of this book, as a professor for my OS class last spring. It was one of the best classes I've ever taken. It's clear that he puts in a ton of effort to explain the concepts clearly. In light of the recent submission about XV6 (http://news.ycombinator.com/item?id=3753216), this book will be useful if you want to dig into the code.