The manual.
One of the Rorschach Tests.
The Psychology Manual is a book that appeared only in "Look Into My Eds." This book includes Rorschach tests (to prove if people are "psychologically normal"). It's famous for containing the Hypnotizing Wheel, which Eddy and the other Eds used to hypnotize the kids all throughout the same episode. The famous wheel also appeared in "The Good Ol' Ed," when Edd was building the Time Capsule and burying the wheel, but the book was not seen in the episode. It's possible that Edd did not want to part with it; after all, when he got the book in the mail, what he was interested in was the manual itself, not the hypnotizing wheel.