THE GIFTED children's illustrator Marcia Brown once observed, in the course of criticizing some particularly garish and overblown picture books, that the aim of illustration is "to express in just proportion the idea within . . . A book starts with an idea, whether or not it has a text, and illustration is at its service." Successful illustration, she maintained, "extends, embellishes, illuminates, but never obliterates the idea."

Picture books have become so expensive of late that some such principle of discrimination is worth remembering. Its obverse, of course, is that a picture book should also be judged on the quality of its text; of what good are apt or striking illustrations if the idea they serve is vapid, pernicious or confusing to begin with?

The Riddle, retold by Adele Vernon, illustrated by Robert Rayevsky and Vladimir Radunsky (Dodd, Mead, $12.95; ages 5-9). Adapted from a Catalan folk tale, The Riddle is an unostentatious gem of a picture book, in which the story itself, Vernon's prose and the illustrations by Russian-born artists Rayevsky and Radunsky are all equally rich and economical. A king, lost in a forest, encounters a charcoal-maker, who refreshes him with roast onions and cold water. The king is struck by the charcoal-maker's extreme poverty but the man is both cheerful and witty, explaining by means of an ingenious riddle how he makes ends meet. Delighted, the king decides to challenge his courtiers with the riddle upon his return. The game of wits continues, with a surprise ending that only goes to reinforce the idea -- common to much good folk literature -- that there is a fixed social order but an individual may rise limitlessly within it on the strength of brains and honesty.

The pictures are stylized, whimsical and uncluttered in the manner of a medieval frieze. Dramatic effect is achieved through adroit use of color -- rich red for royalty, drab browns and grays for the charcoal-maker's family and the barren central Spanish landscape -- but it is the focus on and careful grouping of human figures that enhance the primary theme of sharp wits at play.

Why The Crab Has No Head, retold and illustrated by Barbara Knutson (Carolrhoda Books, $9.95; ages 4-7). Getting up an authentic folk tale as a picture book does not, however, necessarily guarantee wisdom, as this version of a story from the Bakongo people of Zaire proves. Why The Crab Has No Head is the simple tale of how a female deity, Nzambi Mpungu, grows tired after a long day creating animals and decides to complete the still headless crab next morning. "Come back . . . when the sun has risen, and I will give you a fine head." Filled with pride and excitement, the little crab invites all the other animals to witness the great event. So what happens? Ever alert to the sin of pride, Nzambi Mpungu brings the crab down a peg or two by refusing to give him a head after all. "And he still walks sideways, only now it is from embarrassment instead of pride."

With its generous, competent, black-and-white woodcuts, this is at first glance an attractive book. Knutson, influenced by primitive African art, has executed some especially fine, animal-filled, geometric-patterned borders. But try reading the story to young children and you will see where it goes wrong. So powerfully do they identify with the "little crab" (the adjective is critical) in his not unappealing excess of joyous pride, that his shocking humiliation at the hands of the smug deity (mother?) leaves them outraged and confused, in no mood at all to discriminate between earned and unearned self-satisfaction. And I can't say I blame them.

Why The Chicken Crossed the Road, by David Macaulay (Houghton Mifflin, $13.95; ages 4-8). This is a kids' book with a difference, in that it sets out deliberately to provoke to thought -- difficult thought -- rather than merely entertain or enchant its young audience. The child, quite as much as the philosopher, thinks Macaulay, may be diverted by the phenomenon of cause and effect, the conundrum of circularity, in our lives. Macaulay is famous for his wonderfully detailed nonfiction books for older readers -- Castle, Cathedral, City, Pyramid and others -- but shows a more playful side here. "One day," his yarn begins, "a chicken ran across a road. This startled some cows, who stampeded over an ancient bridge, causing it to collapse onto a passing train" -- and a chain reaction of increasingly improbable and hilarious consequences is triggered. Yet Macaulay controls and directs the turmoil just enough that we are able to finish up with the same chicken, the same cows and ancient bridge, and this tantalizing half-sentence: "Which is why." And so we are poised to begin the whole crazy romp all over again. Or are we?

The tempera illustrations are big, poster-bright, cartoon-like and so energetic that one sometimes wants to hold the book three feet further away to avoid a kick in the eye. Do they threaten to obliterate the idea? Not at all. With their teasingly foreshortened or skewed angles and perspectives, they certainly reinforce the qualities of curiosity and originality (let's try looking at the world this way) which stamp all David Macaulay's books, including this one.

The Z Was Zapped, by Chris Van Allsburg (Houghton Mifflin, $15.95; all ages). Van Allsburg, another contemporary master (Jumanji, The Wreck of the Zephyr, The Mysteries of Harris Burdick, The Polar Express and others), is a man with a passion for new directions. None of his books trades on earlier successes; many of them, as a result, take some getting used to, or living with, to be fully appreciated. This alphabet book to end all alphabet books (the fate of the Z confirms that) is a case in point, though I should say that my two young sons, aged 7 and 5, experienced no such hesitation, practically tearing the book out of my hands in their eagerness to find out what was going to happen to the next letter. Alphabet books don't usually have this effect on them.

What happens in this "Play in 26 Acts" is that each of the letters, appearing in turn on a draped and sinister stage, suffers some mishap or injury, from the merely inconvenient ("The S was simply Soaked") to the macabre ("The N was Nailed and Nailed again") to the fatal ("The F was firmly Flattened," "The Q was neatly Quartered," "The E was slowly Evaporating"). No wonder the J is Jittery. The drawings, done with Van Allsburg's inimitable, shadowy, grainy pencil, can be blood-curdling: look at those gloved hands coming quietly out from behind the curtains to Kidnap the K. And the letters themselves, Caslon typeface carved from wood, are almost animate -- victims, martyrs and heroes all. The educational quotient, though rightly secondary, is enhanced by the trick of having the identifying sentence follow rather than face a given letter. Thus "The B was badly Bitten" faces not B but a shredded C; the reader has a chance to name C's injury before turning the page to confirm that it has indeed been Cut to ribbons.

I never thought I'd bother with alphabet books again after Anno's Alphabet (1975), but this is one to stand on the shelf beside that small masterpiece. :: Elizabeth Ward writes frequently about children's books.