Possum Magic , by Mem Fox (Abingdon, $11.95; ages 3-6) "Picture books," says Australian children's author Mem Fox, "are so . . . hard. You use so few words you must choose the very best." Sparse text, a clean plot line, an obvious climax and Julie Vivas' delightful, star-sprinkled illustrations are the ingredients Fox uses to brew Possum Magic, a spirited if somewhat nationalistic romp of a tale that, since its publication in 1983, has become far and away the best-selling contemporary Australian children's book.

To keep Hush the Possum safe from snakes, Grandma Poss -- famous for her bush magic -- makes her invisible. "What adventures Hush had!" But when Hush wants to become visible again, Grandma's magic inexplicably fails. The trick, it turns out, is to eat the right kind of food, "people food -- not possum food," and so the pair set off on a rollicking tour of Australian state capitals and cuisine, sampling all the classic dishes from Vegemite sandwiches in Darwin to desserts like pavlova in Perth and lamingtons in Hobart (map and glossary provided). The magic works, of course, and the starry-eyed possums and their bush friends hold an annual feast from then on "just to make sure that Hush stayed visible forever."

The Friends of Emily Culpeper , by Ann Coleridge (Putnam, $12.95; ages 3-5). The beauty of this equally understated picture book undoubtedly lies in Roland Harvey's pen and watercolor illustrations, which suggest in their detailed drollery a cross between Anno and Quentin Blake. "In a quiet green valley," the story begins, "is a village," in which indeterminate place lives a seemingly amiable old lady named Emily Culpeper. Emily, it turns out, is a witch. But she is not a lonely witch, because of all the friends who come to visit her -- the milkman, the postman, the plumber -- whom she miniaturizes and keeps in various glass jars around the house. One day the local policeman comes to release them. No problem. But it is what then happens to the policeman that provides the kicker, at once comical and chilling.

The Castle Builder , by Dennis Nolan (Macmillan, $12.95; ages 5-8) More distinguished illustrations, this time from the pen of Dennis Nolan, who accompanies his simple tale of a boy, a sandcastle and a make-believe game with photograph-like, black-and-white studies, hand-drawn with amazing precision, dot by dot, to suggest actual grains of sand. The plot itself, featuring young Sir Christopher, Builder of Castles, a gigantic red dragon, Black Knights and a marauding sea, is true enough to the spirit of little boys' action play, despite its slightly stilted prose, to capture -- if not necessarily inspire -- juvenile imaginations. But the pictures, all beautifully composed, dramatically angled "shots" of towers and turrets, shadows and sunshine and spiral staircases, dungeons and dragons, are what both children and parents will remember.

Mars

and

Uranus , by Seymour Simon (Morrow, $13 each; ages 4-8) Seymour Simon continues his unparalleled series on the heavenly bodies (including so far Jupiter, Saturn, The Sun and Stars) with a look at two more of the planets and what scientists have learned about them since the historic Mariner, Viking and Voyager journeys of the past two decades.

The ancient Romans named Mars after their god of war because its eerie reddish light reminded them of blood. H.G. Wells in The War of the Worlds depicted murderous Martian creatures invading Earth. Science fiction writers and astronomers have vied with each other since in imagining possible forms of life on Mars. But in the 1970s, when Mariner and Viking spacecraft finally reached Mars, "they found no canals . . . no cities, no intelligent Martians, and no life at all on the planet" -- nothing but a dusty, windswept landscape of rust-red rocks, deep-cratered volcanoes and barren valleys, shown here in 16 spectacular color photographs from NASA's files.

Uranus, the seventh planet, almost 2 billion miles from the sun, remains a much more mysterious proposition, even with the revelations provided by Voyager 2's cameras in its flyby of January 1986. In his always lucid, elegantly cadenced prose, Simon comments on a selection of these remarkable photographs: here are Uranus's superheated, watery atmosphere; its blue-green clouds of methane gas; and detailed glimpses of its five largest moons, including Miranda's "racetracks" and gray Umbriel's puzzling bright ring, dubbed by scientists "the fluorescent Cheerio." These are dazzling books.

The Old Woman and the Willy Nilly Man , by Jill Wright, illustrated by Glen Rounds (Putnam, $12.95; ages 4-7) What child could resist the opening of this folk-tale, inspired -- so the author says -- by stories of the Willy Nilly Man she heard as a child growing up in Oklahoma and the mountains of neighboring Arkansas: "Once upon a time in the woods, a little old woman lived all by herself. One night she was gittin ready for bed. She brushed her teeth, took off her day clothes, put on her night clothes, and set her little shoes on the floor by the bed. Then she did something mighty peculiar . . . She hobbled outside and got two big rocks." And you should see the picture here: the little old woman, fierce face front, feet planted wide apart, looks as if she is about to pitch those rocks right out of the book at the reader.

Hooked? Read on. The rocks are to be put on the old woman's shoes which, for the past 10 nights, have plagued her by dancing and carrying on till daybreak. When the rocks fail to keep the shoes in check, the little old woman turns to the scary old Willy Nilly Man for help. ("And he's got a beard down to his knees. And all kinds of weird things live in that beard. Spiders live in there and lizards. And sticks and grease and pieces of things he's eaten just hang around on it.") Suffice it to say the ending is just as satisfactory as it is unexpected.

The Monster That Grew Small , by Joan Grant (Lothrop, Lee & Shepard, $11.75; ages 4-9) There never was a more fearful child than Miobi, terrified by his uncle's tales of monsters lurking in the forest: crocodiles and shadow-concealed snakes and great hairy spiders. But "one day, when he came to the most frightening part of {the} path, he heard a voice crying out from the shadows of the darkest trees." The voice, it transpires, belongs to a kind of magical Hare who makes his home in the Moon but who is caught in a tangle of forest creepers and even more frightened than Miobi. Grateful for Miobi's help, the Hare leaves him the gift -- or trick -- of courage, enabling him to overpower the Monster that threatens the village (which is the same as the monster of fear in our hearts.) "This is very curious indeed," Miobi reflects. "The farther I run away from the Monster, the larger it seems, and the nearer I am to it, the smaller it seems."

Joan Grant's version of this lovely Egyptian folktale was originally published in England in 1943. (Another of her stories, The Blue Faience Hippopotamus, received a Parents' Choice Award on its republication in 1984.) The full-page illustrations by Jill Karla Schwarz -- dominated by mid-green and deep purple-blue and terracotta -- are stylized in the manner of ancient Egyptian friezes, complete with borders of geometrical designs, but vibrant also with the energies of other traditions, from African to Oriental.

Elizabeth Ward writes frequently about children's books for Book World.