I agree that The Phantom Tollbooth is especially useful for multiple reads. I read it when I was 8, then when I was 10, then 13, and again when I was 15. It's a wonderful example of a multi-layered children's book that adults can appreciate on an entirely different level than kids.

Also, it taught me much of the basics of good writing, in a way that primed me for reading, eg, Orwell's essays on the use and abuse of the English language.