Gruelle also wrote a stand-alone book called The Magical Land of Noom, which is heavily inspired by The Wizard of Oz (Gruelle absolutely loved L. Frank Baum). I haven't read it yet, and it's pretty difficult to find. D: Gorgeous illustrations though.

There's so much wordplay to catch in Phantom Tollbooth when you're older! I think it's a treat for linguist and math nerds alike. Something which I'm sure even King Azaz the Unabridged and the Mathemagician would agree with. ;D