IT MAY BE years before the American children's book industry realizes what it has lost with the recent death of Arnold Lobel. Having illustrated nearly 100 titles (some of which he also wrote), Lobel was sometimes taken for granted within the trade. The critical response to his work was oddly ambivalent. He did join Wanda Ga'g, Robert Lawson and William Steig as the only illustrators to be honored by both the Caldecott committee for pictures and the Newbery for writing. Yet Barbara Bader ignored him in her mammoth survey of the 20th-century American children's book industry, American Picturebooks from Noah's Ark to the Beast Within (1976). Surely Arnold Lobel is one of the few in the field who will be remembered into the next century.

Lobel served a long but fruitful apprenticeship as a picture-book artist. On graduating in 1955 from Pratt Institute (where he studied with Richard Lindner and met his wife Anita, also an illustrator), he found himself unable to break into the then tiny field of children's books. Instead Lobel went into advertising and, later, trade magazines, which he hated. He also painted portraits. By the late 1960s, however, after a period of rapid expansion in the field of children's books, Lobel had published several easy readers and original folktales, complete with fairly conventional illustrations; it was also rumored at the time that he was being groomed as Maurice Sendak's heir apparent, with his publishers having him illustrate the type of book -- Charlotte Zolotow's The Quarreling Book (1963), Judith Viorst's I'll Fix Anthony (1969) -- that Sendak had long ago abandoned.

In truth the two artists had little in common. While Sendak delved into the darker aspects of his psyche, Lobel's books for children remained sunny, warm, even cosy. "I'm really a very domestic kind of person," he explained. "There's a lot of furniture, a lot of accoutrements of the home, because that's what I am. I'm really not much of a traveller or wanderer or adventurer and I think that feeling certainly comes into my books." What Lobel offered young readers was reassurance. Unlike Sendak, Lobel rarely worked with collaborators of the first rank and generally avoided the classics. When asked to illustrate Aesop's Fables, sweet-natured Lobel was truly shocked by their content. "I found dogs tearing sheep into pieces," he admitted. "I found deer being chewed to bits by lions. I came upon harsh cruelties and bitter ironies of every sort."

But a guffaw is as good as a gasp. Lobel could illustrate Jack Prelutsky's tongue-in-cheek Nightmares (1976) with uncharacteristically comical Edward Goreyesque drawings. In fact his sensibility was more suited to nonsense verse than to fairy tales; it is a pity he never attempted a complete Edward Lear. He did produce the best modern interpretation of The Comic Adventures of Old Mother Hubbard and Her Dog (1968). As charming and unpretentious as an 18th-century chapbook, it is a quiet masterpiece of his vision of the picture book as a miniature play. "There is a little world at the end of my pencil," he explained. "I am the stage director, the costume designer, and the man who pulls the curtain."

But it was not until 1970, with the publication of Frog and Toad Are Friends, that Lobel's excellence was finally acknowledged. The book was immediately hailed as a classic. Now in complete command of his powers as a picture-book author and illustrator, Lobel followed this success with more ambitious productions. Some were disappointing: Lobel was by nature a miniaturist, so the more grandiose his intention, the less satisfying the book. His first full-color book, The Man Who Took the Inside Out (1974), was a bad imitation of Lear. His most honored work, Fables (1980), is a suite of what are perhaps his prettiest pictures, tasteful, classic and conservative. Yet Lobel lacked the anger to be a great fabulist.

When relaxed, however, Lobel could produce some of the most delightful children's books of recent years. Being light-hearted is not the same as being lightweight. His lively watercolors for Doris Orgel's nonsense verses Merry Merry FIBruary as well as those for his own Gregory Griggs and Other Nursery Rhyme People (1978) are as good as anything he has done. The latter, a boisterous collection of little known Mother Goose rhymes, contains many of Lobel's most spirited designs, making good use of his muted, appropriately 18th-century, Thomas Rowlandsonian palette.

He followed Gregory Griggs with two more generally overlooked picture books, a remarkable collection of original limericks, The Book of Pigericks (1983), and a nonsense verse exercise of his own, Whiskers and Rhymes (1985). As a writer, Lobel also helped bring out the best in his wife Anita by providing the texts for two of her finest efforts, On Market Street (1981) and The Rose in My Garden (1984).

He and Jack Prelutsky produced the splendid Random House Book of Poetry for Children in 1983; an ambitious companion volume, The Random House Book of Mother Goose, appeared last year. Lobel admitted, "I hadn't been too pleased with the way Mother Goose had been handled by its recent interpreters . . . so polite, so genteel, so well behaved. My concept of Mother Goose is just the opposite: bawdy and naughty." But that was just bluff. Lobel softened the crudities of the 18th-century rhymes and produced arguably the jolliest Mother Goose of all time.

But Arnold Lobel will no doubt be remembered principally for his four Frog and Toad books, among the few easy readers of any lasting literary distinction. The author knew that the first, Frog and Toad Are Friends, was a landmark in his development as a writer. "When I first started writing," he said, "I would begin by writing stories for children that were really outside my own feelings." With the Frog and Toad stories, Lobel looked deeper within himself, drawing on his adult preoccupations that he then tilted towards child's concerns. Each tale contains some small dilemma -- a lost button, a bowl of tempting cookies, Toad's modesty when swimming -- always deftly solved with simple common sense and the gentlest childlike humor.

What holds these fragile incidents together is the strong friendship of Frog and Toad. They are among the great teams in juvenile literature, as memorable as Pooh and Piglet or Charlotte and Wilbur. Frog is like a wiser older brother to silly little Toad. Lobel wrote other clever early readers (notably Mouse Tales, 1972, and Mouse Soup, 1977), but they are mere whispers of the Frog and Toad stories. More than any of the other characters he created, Frog and Toad are children's friends. Lobel's pictures in soft mouse brown and leaf green beautifully define their personalities and their world; typical of Lobel's tender care with detail is the illustration of absurd Toad in his bathing suit with all the other animals, even the dragonflies, laughing hysterically.

The last tale in Fables, "The Mouse and the Sunshine," describes how a little mouse suffers many hardships before settling quietly by the seashore to watch the sun rise. The moral seems to summarize its author's career: "All the miles of a hard road are worth a moment of true happiness." It is to be hoped that Arnold Lobel found that moment himself.

Michael Patrick Hearn's most recent book is "The Porcelain Cat," illustrated by Leo and Diane Dillon. He is currently writing a biography of L. Frank Baum.