I feel the same way now with "Gliding Flight". The website says its a "children's book", but don't believe it. I consider myself pretty good at spacial relationships, but this book had me scratching my head for a good 20 minutes on some of the folds he uses. It helps that the outer 1" of the page doubles as a flipbook showing animations of some of the harder folds.
The other great thing about this book is that none of the designs require any cutting. Iv'e seen others that have you cut individual wings or produce tail fins by cutting the paper. This is supposed to make the planes look cooler without actually having to fold them right. It usually produces crappy planes that don't fly.
These planes fly, and 2 out of 2 cats at my house agree, they're fun to play with!