THE WRITING of fantasy or even science fiction is no new departure for William Kotzwinkle. The sweet, ironic simplicities with which he charmed readers in ever greater numbers through the 1970s -- winning the National Magazine Award for Fiction twice along the way -- included such sf texts as Hermes 3000 (1972) and Doctor Rat (1976). They were black little fables, and rather up- market.

E.T. on the other hand -- at least as created by Steven Spielberg in the celebrated money-spinning movie -- is a cute, reptilian alien who appeals to markets with both blue and white collars. It was therefore a surprise when the book of the film, E.T.: The Extraterrestrial (1982), a surefire bestseller if ever there was one, was given to the intellectual Kotzwinkle to write rather than to one of the popular-market hacks who regularly perform such mundane book-of- the-movie chores.

Now Kotzwinkle has entered yet another phase, for E.T. The Book of the Green Planet (unfilmed as yet) is a sequel to a book-of-the- film, although it is a million miles from say, the Star Wars spinoffs in style.

This sequel, according to the credits, is based on a Spielberg story, but the style and particularly the humor seem to owe little to Spielberg. This may be, artistically speaking, a net loss rather than a net gain. Lovers of the film are likely to be baffled by the exotic whimsy of Kotzwinkle's plot, which feels wholly unlike its hugely popular forerunner, and they may be perturbed by the apparent discrepancies between this book and the original movie. For example, much of the film's resonance came from the friendship between a lonely Earth child who missed his runaway father, and what appeared to be a lonely alien child who dreadfully missed his mother. It is upsetting to discover now that E.T. was no child; he is a dry old doctor of botany with an aged soul. Lovers of Kotzwinkle, on the other hand, may wonder what has happened to his tough mindedness. Sentimentality used to lurk almost invisibly at the fringes of his vision, but now it is squarely center frame, a valentine stamped out of marshmallow. For this, in part, is a book about the redemptive power of love. Tears of joy and glowing hearts abound.

When E.T. gets back to his home world, the Green Planet (where he learned his outstanding botanical skills) he finds himself not treated as a hero, as he expected, but in disgrace for getting too involved with the natives of Earth. The Green Planet is a world of amazingly varied plant life, and as an sf creation is not at all like anything in, say, Frank Herbert or Robert Heinlein. It is, however, very reminiscent of the world of Antoine de Saint-Exup,ery's The Little Prince (1943). But the inventiveness is lamer than that of the eccentric Frenchman, and the Green Planet is, to use an appropriate English word, twee. All the multifarious, cute detail about Jumpers and Flopglopples and spaceships grown from turnips is somehow repetitive. (As to the science content, not untypical is Kotzwinkle's belief that the square root of minus one is an equation.)

E.T. misses the Earth child Elliott, who had become his best friend. So plangent is his longing that his vibrating heartstrings are able to send little telepathic replicants of himself all the way back to Earth, across the galaxy, bearing messages for Elliott. Sadly, however, Elliott is not receptive. Now a teen ager, he has only one thing on his mind, the pangs of calf love, and he is deaf to E.T.'s call. My friend is in danger, E.T. muses. He is about to become the most terrible thing of all. He is about to become Man. Well of course he is, you soppy little alien! We all have to grow up, and it's not all bad. But perhaps it is only on Earth that adult life is barren? Elliott, we are told, is about to become just another dull pattern of conformity. It is all very well for E.T. to shudder at the adult lifestyle of Californian suburbia, but one hardly knows from what perspective he is able to make this patronizing judgment, for the Green Planet is itself an Orwellian Utopia of conformity. Even misery is forbidden, and eradicated by the Contentment Monitor. It is certainly strange that the two planets, apparently intended to be in dramatic contrast, should emerge (unintentionally?) as similarly frightful.

IT DOES NOT take a very perceptive reader to see that underneath the layers of vivacious surface color, and then of marshmallow, is a brooding despair -- a contempt, in fact, for the whole adult world. (Even the world of childhood pictured by Kotzwinkle is more silly than wonderful.) Spielberg knew better in E.T. than to show contempt for his characters. He poked fun at suburbia, but he also respected the gutsiness and independence of some of its inhabitants, including Elliott's mother. Kotzwinkle sees her as aimless, manipulated, and in matters of the heart very nearly a cretin.

The running gag of the book is E.T.'s obsession with the trivia of Earth life, most of which he barely understands. Most of this material is more flippant than witty; it has the ring of satire but no satirical function. Hardly a sophisticated aper,cu is the plot point that E.T.'s heroic escape to Earth in his homegrown spacecraft is largely because he is hungry for candy. He yearns for football helmets and bicycles and hopes one day to become a Drop Out. He bribes the lordly mineral-eaters with the promise that if they come to Earth with him he will make them Burger Kings. He speaks the language so well, he says, that he wil be able to communicate with such important functionaries as astronomers and nerds.

Mildly amusing, but is it heresy to suggest that in some areas Kotzwinkle has a tin ear? Many of the colloquialisms that E.T. so mangles are more adult than childlike and since on his previous visit he had almost no contact with adults, where did he latch on to phrases like big bucks and Fat City? All this vocabulary is seen by E.T., ironically, as rich in nuance, but then again, he may be right.

Kotzwinkle's own language is not quite that of the Gifted Bard that The New York Times once called him, and certainly falls short of the spiritual beauty claimed in the cover copy. He is more of a pop crooner than a bard these days, it seems, and his spiritual exploration is about at the level of doctor- and-nurse romance, as in the ersatz profundity of his "love is just a speck of dust in the ages. . . but I believe it is the only treasure." Oh dear.