A FISTFUL OF FIG NEWTONS By Jean Shepherd Illustrated . 265pp. New York: Doubleday & Co. $14.95 By MARTIN A. JACKSON It was 1957, and I was wide awake at 2 A.M., hopelessly trapped, listening to Jean Shepherd's night-long radio show. In the years when Eisenhower was in the While House, Jean Shepherd was a radical in the best sense of the word. He switched on light bulbs in the heads of a whole generation by simply explaining America to us. For five hours at a time, he beamed his erudition and good humor out over the East Coast, and we began to understand this odd nation a bit more. I still think of him as a matchless radio artist, but he's a fine writer, too, maybe one of our major humorists. Please note that a humorist is not a comic: Shepherd doesn't tell jokes; we aren't talking of Don Rickles here. What Jean Shepherd does is uncover absurdities and make us look cleanly at our own times. It's no easy trick. "A Fistful of Fig Newtons," Shepherd's third book, confirms my view. He writes about cars, the Army, summer camps, the sixth grade, his mother's meat loaf and beer-burping college kids in the Holland Tunnel - in short, about our life and times, the ordinary bits and places that every American recognizes and that are loaded with meanings for us all. He describes ordering, for example, "a rich slab of the Mother Food of New Jersey. Known to the pizza aficionado as a "Full-tilt Boogie,' it had everything: anchovies, sausage, green peppers, double cheese, onions, and the greasy thumbprints of Vinnie himself." Over the years Jean Shepherd has discovered a way of transposing his radio persona into print. All of the pieces in "A Fistful of Fig Newtons" have an aural dimension; they sound like Jean Shepherd, in rhythm, vocabulary and structure. It isn't necessary, though, to be a veteran radio listener to appreciate them. The American inner landscape Shepherd depicts will be familiar to anyone born after Herbert Hoover's Administration. His tale of the Great Ice Cream War, for instance, of how Mr. Leggett finally beat out the competition, the upstart Happy Cow, by just for one night giving away ice cream cones, is a story of endurance, bravery and existential choice, written in the juicy vernacular that Jean Shepherd has made his own. So is the epic of Ernie, Shep's G.I. buddy, who gambled for mighty stakes one hot night on a troop train and lost. "Is he out there yet," Shepherd wonders, "a haggard wraith living on berries and dead frogs?" It is Shepherd's gift and his burden to be addicted to America. He's a piece of flypaper upon which the dust and flotsam of this peculiar civilization have been gathering for years. The sound of his old man's Pontiac revving up In the driveway? Shepherd remembers. The look of a college football team? Shepherd knows: "Big AI was wedge-shaped; pure sinew, gristle, and covered with a thick, bristly mat of primitive fur. Numerous broken noses had reduced his nostrils to blow-holes." Shepherd understands the first requirement of the humorist: affection for his subject. He's part of a tradition that includes George Ade, Robert Benchley, James Thurber and even Mark Twain. His wit is a kindly inside needling, a fond reminiscing about the embarrassing moments and quirky habits of people we've all grown up with. He's also kind to New Jersey - surely the mark of a good man. Jean Shepherd has concentrated on writing for the past few years. It would be nice, though, to have him back on radio, too, to hear that rich Indiana voice on a clear 50,000-wint signal once again. I'd like to listen to him ramble some more about the unforgettable feel of a '39 Chevy on a hot evening. In the meantime, we can read Jean Shepherd, and that is a delight.