1. It's not replaceable by some other title in the same genre, or by the same author, or on the same topic.
2. Every few years (or months or whatever) you feel a restless urge--or a driving need--to reread it.
3. There's something in or about the book that was meant personally for you. No one else can understand it just the way you do.
4. Even if you forget about it for years . . . when you pick it up again, everything is perfectly familiar.
5. No matter how often you read it, you are never disappointed. And always surprised.
