What will I be when I grow up,"" asks Mayer's homely-cute Little Monster, and so his grandfather takes him round to a circus, a medical center, a farmer's market, an airport, the moon, and other places where Little Monster can try out road building, TV work, science, crafts, the fast-food business. . . much as Mayer himself tries on different styles. Having worked his way through Sendak he is now playing Richard Scarry, this time with a bustling, label-filled double page for every field of endeavor. Mayer throws in some dopey one-liners (""Take me to your monster"") and adult recognition gags (a submarine captain asks ""Have you seen the White Whale,"" Gene Shalit pans a movie on TV); but even though the pages are a-jumble with monsters all busily at work, they convey no sense of vitality.