Using this textbook right now for my grad-level OS class (I'm a student). Definitely one of the most readable textbooks I've ever used -- the authors actually try to impart some personality into the writing. I'm not 100% sold on how they divide the book into 3 parts -- virtualization, concurrency, persistence -- since not everything in an OS fits precisely into one section (e.g., multi-core scheduling is in virtualization but could also fall under concurency). Still, for the price (free/$10 enhanced PDF/$22 softcover) I'd heartily recommend this book to anyone, and I hope it quickly becomes the standard OS book.