"I didn't hear a thing," George said. "It must have been --."

Her words ended in a surprised gasp, for a low groan seemed to issue from the very floorboards. "Did you hear that?" she whispered apprehensively.

"What could it have been?" Bess shuddered.

"It sounded like someone in pain," Nancy replied. They huddled together, listening intently. A moment later they heard the strange noise again -- a loud, unearthly moan.

"It's a ghost," Bess insisted, her teeth chattering with fright.

"There are no ghosts," Nancy returned firmly. But it took all her courage to add, "I'm going down to the cellar to find out just what it really is!" -- From "The Message in the Hollow Oak," 1935 by Carolyn Keene

This year marks a half-century of the floorboard groans and creepy cat burglars who plagued but never defeated Nancy Drew, the daring girlhood mystery book idol who should be 68 by now. But in her world -- the world of the zippy blue roadster and villians who disturb (only temporarily) the shaded streets of River Heights -- she is still, thank goodness, just 18. t

Always, there was the wonderful cliffhanger at the end of each chapter. Millions and millions of little girls would rip frantically through to the next page, maneuvering the flashlight under the bedcovers. It was Nancy Drew, blond teen-aged detective, sleuthing her way through the bloodcurdling screams of mystery No. 12. It was heaven.

There was a party for her in New York last night, or more specifically, for her author/mother Carolyn Keene. Her real name is Harriet S. Adams, a blond and curly-haired 87-year-old great-grandmother who is even now dictating mystery No. 60 on the tape recorder in Maplewood, N.J.

"Nancy is like a daughter who is very close to me," she's said. "I guess we have grown closer as the years have gone along. Being a fictional daughter, she does exactly what I tell her to, or rather, let her do."

Last night, Adams took the evening off to hold court for nearly 500 Nancy Drew fans at Harkness House off Fifth Avenue. The birthday party was hosted by her publisher, Simon & Schuster, and with it came a blue roadster parked outside, a blue roadster cake inside, one dark and menacing cave, sinister fortune tellers, and a crowd that included some terribly serious contemporary writers. Some of them maybe got their first stirrings of impending authordom while sneak-reading Nancy Drew inside the grammar book in language class.

Among the authors was Susan Brownmiller, who has written about rape but drooled over Nancy Drew in her pre-feminist stage. "She was a winner," Brownmiller remembers."I mean her friend George dressed like a boy and Bess was fat, so you didn't want to be her. Nancy had the car, Nancy had the boyfriend."

To get to the main part of the party, which sprawled on four floors from a soda shop to a dance floor to a table with perhaps nine kinds of chicken liver, guests had to traipse through a cave full of chilling, pre-recorded screams. The only defense was a mini-flashlight, given to each cave stroller as a party favor.

Once through, they were greeted by a youngish New York crowd, the youngest possibly being Nancy Drew herself. She was really Nancy Ewan Robey, a records manager at a New York ad agency, but she certainly acted like the perfect girl detective, especially during the denouement.

For, of course, the party had a plot. Every hour, a loudspeaker broadcast a clue; there were four of them, all in rhyme, and almost no one paid any attention. But around midnight, the butler offer the bass player a creme de menthe, which he declined; the butler brushed the band member's lapels, and in so doing, removed his emerald stick pin and dropped it into the liqueur. a

Then the butler sort of pussyfooted across the floor. Halfway, Nancy Drew and Ned Nickerson surrounded him and cried, "The Butler did it."

Guests at the party were actresses Blanche Baker and Carol Lynley, critic John Simon and authors Nancy Friday and Judith Rossner, as well as Pat Kennedy Lawford and Fran Lebowitz. She, for one, said she still reads Nancy Drew.

"I haven't stopped," she explained, recalling an eye operation at age 8 that left her bandaged and therefore blinded for two weeks. Her mother, she said, read her -- aloud -- one book per day at the hospital. "Carolyn Keene is my favorite author," she said.

Lots of the other girls who once read Nancy Drew have graduated to Joan Didion and grocery coupons. Still, they're grown-ups who, if they don't exactly remember "The Password to Larkspur Lane," do remember they read it that long, sweet summer at camp. Nancy Drew books are the kind of books that jumble together with memories. Today, those memories may be worth more than the three-hour girlhood escape to the idyllic town of River Heights.

And then, too, Nancy may have instilled in them, in her own prim way, the idea that a girl could do it, and do it by herself. Superwoman? She needed no super powers, just her own blend of curiosity and guts. There was never any Charlie needed to set her angel in motion.

Which may explain why so many of her fans have gone on to scale heights Nancy herself would have paled at. Among those fans:

Barbara Walters. "Seems to me I read all of them. It was escape. When you had some time to yourself, you could curl up in a chair in a corner with Nancy Drew."

Joan Mondale. "I was crazy about them."

Beverly Sills. "I loved them. She had a car and she was pretty and to us kids in Brooklyn, that was sophisticated."

Ruth Bader Ginsburg, Columbia Law School dean. "I liked Nancy Drew, yes. She was adventuresome, daring and her boyfriend was a much more passive type than she was."

Liz Carpenter, assistant secretary of education. "I loved the way she was always zipping around in her trusty blue roadster. I wanted to be just like her."

They were halfway around the deck when Bess suddenly cried out, "Look! Look!"

A bright object was descending from the sky at lightening speed.

"It must be a meteor!" Nelda said, excited.

"And its going to land on this ship!" George exclaimed. "We'll all be killed!" -- From "Mystery of the Brass-Bound Trunk," 1976, by Carolyn Keene

Many of today's more sophisticated little girls read, with caught breath, of the wonderful puppy love and agonizing acne in Judy Blume books. These characters fight with parents, and hold hands, and kiss, and sometimes even more.

Not Nancy. Through 50 years and 70 million 7- to 13-year-olds, she has kept her squeaky-clean boyfriend. Ned Nickerson, interested at an awesome distance. Only recently, in the later mysteries, have their lips touched. And only recently has Nancy herself switched from blond to strawberry blond and from sweater sets and matching handbags to jeans and even shorts, although it does have to be horribly hot before Harriet Adams will allow such a thing. And in Nancy's half-century on the job, she has aged only two years, from 16 to 18, and that only so she'd be able to drive in all states.

Nancy has not, however, switched her life sytle from the pleasant, upper-middle-class existence in River Heights, U.S.A., a life style marked by a plump, friendly housekeeper and an amazing absence of blacks, Jews, Italians, Chinese and other minorities.

Her atavistic environment lives on today, along with her upstanding morals and strait-laced behavior.

Nancy has, said Adams, "good, old-fashioned, sound, moral ideas."

Nancy Drew is actually the creation of Harriet Adams' father, Edward Stratemeyer. He gave birth to the Bobbsey Twins, Tom Swift, the Rover Boys and the Hardy Boys and was, among other things, Horatio Alger's editor. In 1930, he cooked up Nancy Drew and wrote the first three mysteries about the poised and proper girl detective. After his death, his Wellesley-educated daughter revised the first three, making Nancy a bit more polite and respectful. (She didn't like, for instance, the way Nancy talked to Hannah Gruen, the housekeeper.)

Nor did she like it when Pamela Sue Martin, the Nancy Drew of the since-canceled television series, posed for Playboy. The mother of Nancy Drew was simply beside herself.

"A travesty," said Adams, aghast. "Just dirty stuff. Pamela Sue Martin posed for the picture 98 percent nude."

And as long as we're on the subject of unpleasantries, there's the matter of this nasty little lawsuit. It's for $150 million, and was filed against Simon & Schuster and the syndicate that owns the Nancy Drew books. It was filed by Grosset & Dunlap, Adams' publisher until she switched last year. They charge breach of contract, copyright infringement an unfair competition.

"I don't see any reason for it," Adams said of the suit. "It's taken a fabulous amount of time, which interrupts my writing." She also allowed that Simon & Schuster, which is publishing her books in paperbacks, is giving them "the promotion they deserve." Then, too, the party; Dunlap had planned nothing, she said, for the 50th anniversary.

Joe . . . tossed a little stone, which hit the crocodile lightly on the snout. At once his jaws opened. Instantly Nancy threw the chunk of beef. Her aim was perfect and the meat disappeared within a second.

Nancy was so fascinated as she watched the reptile that she failed to retreat. Suddenly the crocodile moved its great tail. In a moment it would hit Nancy hard and injure her!

"Look out!" George shouted. From "Mystery of Crocodile Island," 1978, by Carolyn Keene.

Harriet Adams, who writes behind a French Provincial desk in the Newark suburb that could double for River Heights, has taken Nancy, since she started in 1930, to South America, Scotland, Africa, Mexico and Europe. In a recent book, "The Thirteenth Pearl," she has even disguised Nancy, somehow, as a Japanese girl on her way to Tokyo.

Adams gets her story ideas from her own experiences and, as she puts it, "on-the-spot" research. An example:

"In Africa," she says, "I saw a baboon about to pluck off a woman's wig and yelled to stop him. I incorporated this into a story in which I let the baboon succeed in order to embarrass an annoying young woman."

No one knows how much Harriet Adams has made from Nancy Drew, and she won't tell. Still, it's widely assumed that she is a very rich woman.

And now, on she goes with mystery No. 60 "The Mystery of the Ancient Mask." What's it about? She won't give a clue.

Nancy . . . had found a sliding panel in the wall near where Glenn was looking and saw a closet. There was nothing in it.

The young detective stepped inside. "Maybe there's some kind of a switch in here," she said to herself and began running one hand over the wall.

Suddenly her fingers felt strange and she tried to pull them away. But she was not able to do so. Her whole hand was drawn against the wall. She tugged harder but still could not move it. Before she could call out to Glenn, the panel closed.

Instinct told Nancy she was in serious trouble. At the top of her lungs she cried, "Help! Help!" -- From "Mystery of the Glowing Eve," 1974, by Carolyn Keene