Picture book by Mercer Mayer, a favorite of mine from childhood. Professor Wormbog collects beasties, and has one for every letter from A to Y (and a nice wall of picture of all of them, including such Mayer creations as the Great Glern, the Peevish, and the Yalapappus) but not a Zipperump-a-Zoo. After looking at the only known picture of one, the Professor goes off on a long search for it but never quite finds one. The illustrations have all sorts of easter eggs, so the book stays interesting because there's always something new to point out.