Dr. Seuss answers the question of what comes after Z in On Beyond Zebra!

A young narrator leads his friend Conrad Cornelius o’Donald O’Dell past the ordinary alphabet and into a set of invented letters, each one built to spell out a creature too strange for A to Z, from the Yuzz-a-ma-Tuzz to the Wumbus. Part of the Classic Seuss series, told in Seuss’s own rhyme and full-color art. ISBN 9780394800844.