Ralph Ellison read from his new book last night at the Library of Congress--the first novel he has written since his first one, "Invisible Man," shook the nation 31 years ago.

You would have thought it was Mark Twain.

Reading in a nervous rush before an absolutely packed auditorium of mostly young, mostly black fans who had filled the place a half-hour early and stood six deep in the hall outside watching on television, Ellison gradually warmed up as the laughter washed over him, and by the end of the hour he was laughing, too, stopping in midsentence, choking over some bit of description, each one wilder than the last.

Though he has been publishing short stories occasionally since that one novel, he hasn't been talking about the new book. More than a decade ago, some 500 pages of it were lost in a fire at his summer home in the Berkshires. But he still had another 1,000. Working steadily seven hours a day, he has amassed what he calls an enormous manuscript. That's about all he will say.

At 69, Ellison lives with his wife, Fanny, in the same New York City apartment he has had for 30 years. He has held professorships and lectured at Yale, Rutgers, the University of Chicago, New York University and Bard College, where for two years he was an instructor in Russian and American literature. He is also an authority on music.

When "Invisible Man" came out, a first novel by an unknown author, it stayed on the bestseller list 16 weeks and won the National Book Award. It has never been out of print.

When interviewers ask if the published stories are to be in the new novel, he says he never quite knows. "It's a wasteful way, I guess, but you discover as you go along."

What the audience discovered last night was a rambling childhood reminiscence by one Cleothus, at 300 pounds the biggest kid in the first grade.

Ellison, somber in dark blue, took calmly the introduction by Library of Congress poetry consultant Anthony Hecht, an old friend, who compared his great novel to "Huckleberry Finn," "The Red Badge of Courage" and "A Farewell to Arms."

Then, taking a gulp of water and putting on his half-glasses, Ellison started to read from a battered hardback ledger.

"Tyree flapped his arms like a rooster and strutted around in a circle and pecks his head back and says, 'Brothers and sisters and grandpappy dodgers, Cleothus is a soft horse apple and a ripe goose egg.' And that started them off again yelling, 'Yeaay! He ripped it! He ripped it! He ripped it! Cleothus ripped it like a fool!'

"Then somebody hit the blackboard with a biscuit soaked in molasses and jam and it smashed all over the big map of the United States . . . but Miss Kindly was glaring straight at me. The woman didn't even dodge. 'Young man,' she said, 'you march right up here and apologize to me and the rest of the class.'

"But before I could move, Jack spoke up in his natural voice which he'd already made as rough and deep as Mrs. Louis Armstrong, 'Why, Miss Kindly, what do you mean apologize, why all Cleothus is saying is that he's full of brown, and that's a natural fact. You don't believe it, sniff him.'

"Which really exploded, even those good little twin girls whose momma wrapped their hair in gingham rags and braided it so tight that they couldn't blink."

At that point the food-throwing started, he said, mostly overripe grapes swiped by Jack from the produce house and doled out to his fellow outlaws.

Getting away from the schoolroom for a minute, Cleothus remarks that "this has always been a bad town for nicknames . . . There were so many big burly guys nicknamed Bear that they had to be identified by the man they worked for: There was Helpless Bear and Milton's Bear and McDonald's Bear and Ruby Lyons' Bear. But Weinstein's Bear was close to notorious. Talk about having a bull in a china shop . . ."

About his teacher: "Miss Mabel Kindly was her name. The woman used to talk real proper, rolling her R's and her I's, and wore her hair in three big buns, with the rats always peeping out in back . . . You'd see her walking down the street like she was carrying a thin-shelled egg between her skinny thighs."

The story, or at least the part Ellison read, never made much progress but meandered from one hilarious episode to the next, from a description of the tricycle tire with which the boys were beaten by the principal (whom they called Blue Goose), raising welts "like an alligator," to an outrageous adventure involving a beached whale, trussed up with wires and lights, about which Miss Kindly made the grave mistake of attempting to lecture.

As the hour ended, Ellison stopped, and the crowd, which had been leaning forward, eyes alight, whooping with laughter at his descriptions--many of them scatological--almost wouldn't let him go. When they did leave the auditorium, it was to line up to shake the man's hand.