SOME BACHELOR uncle will no doubt discover Roald Dahl's new collection of stories in a bookstore, on the shelf marked Juvenile, and intend to give it to his favorite nephew for Christmas. He'll remember Charlie and the Chocolate Factory, James and the Giant Peach, and, seeing The Wonderful World of Henry Sugar and Six More, think it in quite the same grotesque and amusing tradition. Dahl, after all, is ever grostesque and amusing.

If the uncle pauses a moment to leaf through the book, however, his nephew may have to do without. Only parts of Henry Sugar are for children.

These are what literary journals like to call fugitive pieces, which is to say pieces that never found a home in other collections. Each is marvelous, in its perverse way, with magically sinister twists that fascinate like a not-quite nightmare. And, then, three of the pieces are true.

"Nonfiction, which means writing about things that have actually taken place, doesn't interest me," says Dahl. "I enjoy least of all writing about my own experiences." Be that as it may, the two bits of autobiography that conclude the book are Dahl at his most inventive and most grotesque.

The stories are neatly arranged, luring the reader on. Unlike most collections, Henry Sugar is hard to put down. The first tale, "The Boy Who Talked With Animals," is what one might expect from Dahl - boy meets sea turtle, boy falls in love with sea turtle, boy goes to sea with turtle. "The Hitchhiker" is an unassuming little amusement about an encounter with a master pickpocket, a "finger-smith" as he likes to call himself.

"The Mildenhall Treasure" is a bit of "new journalis" wittten before the term was invented (ther seems to be an awful lot of that around): a story of two men who discovered one of the greatest ancient treasures of England, and lost it.

Well, by now the book has got you. And it takes a turn. "The Swan" details the unparalleled viciousness of stupid young boys, with images as sadistic as a reader is likely to encounter.

The title story lightens things up again, telling of a rich man, Henry Sugar, who spends his idle time learning the secrets of a yogi until, finally, he is able to see without using his eyes and read through the backs of playing cards.He uses the skill to a worthy end.

Then there is the autobiography.

In "Lucky Break" Dahl tells how he became a writer, and takes the reader all the way back to the beginning, to his days in a couple of hideous elite English schools - the canings and random, relentless persecution, the stifling snobbery, the petty rebellions. He seemed a hopeless case, at least by his own and his teachers' accounts. There wasn't much thought of his becoming a writer.

But World War II changed him. His gruesome experience as a fighter pilot and his injuries led to a tour of duty in Washington, where, of all people, C. S. Forester discovered him.

Dahl's first bit of writing, put on paper at Forester's request then for warded untouched to The Saturday Evening Post, is an eerily understate account of being shot down over Egypt and nearly burned alive. "A Piece of Cake," its called.

"My face hurt most. There was something wrong with my face. Something had happened to it. Slowly I put up a hand to feel it. It was sticky. My nose didn't seem to be there."

This, certainly, is grimmer than Grimm. Not for a child, barely endurable for an adult.

But after our bachelor uncle has stood enthralled near the juvenile shelf for a couple of hours, he will probably decide to buy the book anyway. He can read "The Bou Who Talked With Animals" and "Henry Sugar" aloud to his nephew. He can keep the rest for himself.