I really like the way he limits his list to 4 books. Most other thought leaders/influencers/CEOs do lists of 10, 20 "hot topic" books that you know they probably haven't read. I find the volume also diminishes the individual importance of each book on the list. I'll have to pick up String Theory (I love that Bill Gates reads DFW) and The Grid per his recommendations.
Whatever I know Gates is a reading machine. I am sure he has read everything he recommends. When Jon Stewart did the Daily Show I always wondered if he had read guests' books because he had pretty detailed questions.
I wonder if in some cases his producers would give him "cliff's notes" for the books.