Meet a cranky wizard; Tomorrow's Wizard, by Patricia MacLachlan. Illustrated by Kathy Jacobi. New York: Harper & Row. 80 pp. $9.50.

Wizards, as we all know, perch in clouds, rattle their wands, and shower the earth with promises. What we don't know -- but what Patricia MacLachlan tells us in her most magical of books -- is that wizards are often cranky, prone to indecision, and blighted with incompetent assistants.

In ''Tomorrow's Wizard,'' a wizard of the very same name bestows five important wishes. To help him are his apprentice wizard, Murdoch, a bumbling innocent with a face ''like a boiled onion,'' and a horse the color of clouds. Together they wed Rozelle, the village shrew, with Ethan, the tender giant; they aid a beauty who wants to be loved for herself; and they help a fiddlemaker find the perfect fiddle. In a wonderful twist on wish-granting, they help the Three-D family (Dreadful, Dastardly, Demon) by transforming a sickeningly sweet child into a lovable brat. Happily, they live unhappily ever after.

''Tomorrow's Wizard'' is a quietly stunning book, filled with poetry and parables. Kathy Jacobi's illustrations perfectly complement Patricia MacLachlan's lyrical storytelling. A book that should stand permanently on every child's shelf.

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