NEW YORK - She isn't real - no more real than the Little Mermaid or Mary Lennox of "The Secret Garden.'' Yet her portrait hangs in the lobby of New York's famous Plaza Hotel. And for years, if you called the Plaza and asked for her, this nonexistent little girl would answer the phone.

As she herself would declare, she is me, ELOISE. Kay Thompson's book, "Eloise: A Book for Precocious Grown Ups,'' has sold more than 500,000 copies since it first came out - Tuesday marked the 40th anniversary. It was an immediate hit, with three sequels as well as Eloise dolls and merchandise. The 6-year-old became one of the first baby-boom children's fads - the smart girl's version of the coonskin cap.

But the dolls have disappeared and the sequels have gone out of print. Only the original "Eloise'' remains, still selling steadily, one of the most sophisticated contributions of the '50s to children's literature - and a character almost as vivid and unusual as her creator.

Yet as its subtitle indicates - and as Thompson insisted, despite the dolls - ``Eloise'' was never intended as a kid's book.

"Kay denies it ever was a children's book,'' says Hilary Knight, the illustrator of "Eloise.'' ``I tend to agree with her - although,'' he adds with a smile, "it was the best thing that could have happened to it.''

Indeed, "Eloise'' is the New Yorker of young girl's books. There is an urbanity, a wit, an independence of spirit not usually found in talking bunnies or sleeping beauties. Notably, after those first faddish years, ``Eloise'' has never been a huge seller. Half a million copies is a lot for a kid's book, but it's small change compared with Dr. Seuss.

"Eloise'' is New Yorker-ish not only in its tone and its somewhat select readership but in its perfect fusion of Knight's graceful, playful illustrations with Thompson's crisp, child's-eye-view prose ("An egg cup makes a very good hat'').

Eloise herself is a Huck Finn for city girls, a devilish Alice in a New York Wonderland of room service and grand ballrooms. "Eloise'' is a day in the life of a "city child'' who lives at the Plaza with her British nanny. Her mother, a wealthy young woman ("My mother knows Coco Chanel''), never appears and no father is ever mentioned.

As a result, Eloise is hardly a polite role model or an ad for traditional families: The impish young girl has the run of the hotel, annoying guests and employees but too well-connected ("My mother knows the Owner") to be thrown out.

She remains one of the few young heroines who is smart, funny, frank, inquisitive and irrepressibly bad-mannered ("Tomorrow I think I'll pour a pitcher of water down the mail chute"). She is the princess who is not P.C. - perhaps the world's only beloved New Yorker.

Eloise gets her irrepressibility, it seems, from her creator. As Kay Thompson herself has said, "Enthusiasm and imagination can carry you anywhere you want to go without Vuitton luggage."

Thompson has led a rare existence: singer; radio producer; choreographer; vocal arranger for Judy Garland, Lena Horne and Frank Sinatra; author; movie actor and media recluse. She declined to be interviewed for this story; her last known print interview was with Rex Reed 23 years ago.

Born in St. Louis in 1912 (the year is disputed), the wunderkind daughter of a jeweler, Thompson was performing Franz Liszt with the St. Louis Symphony when she was 15. She went to California to teach diving but wound up singing with the Mills Brothers. Eventually, she worked with choreographer Robert Alton as a songwriter-vocal arranger on such classic MGM musicals as "The Ziegfeld Follies" and "The Harvey Girls." Songwriters Hugh Martin and Ralph Blane ("Meet Me in St. Louis") credit her with giving them their start in the music business.

There were two marriages - one to bandleader Jack Jenny, the second to producer William Spier - but she was single again by 1947.

That year, together with Alton, Thompson developed a nightclub act with the Williams Brothers (including Andy Williams) that became a sensation.

"It was toward the very end of that period of supper clubs," Knight says, "and they were very big in those circles, in big-city clubs."

It was during rehearsals with the Williams Brothers that Thompson hit upon her most famous creation. When confronted about her tardiness one day, she responded, "I am Eloise, and I am 6."

It became a game, a little-girl voice that she would fall into during conversations. In years to come, it was, of course, Thompson who answered the phone at the Plaza when fans of the book asked for Eloise. And as late as 1988, director Francis Ford Coppola tried to persuade Thompson to let him adapt ``Eloise" for the 1989 anthology film "New York Stories." (He and daughter Sofia Coppola eventually developed the derivative "Life With Zoe" segment). According to Knight, Coppola spoke to Thompson for several hours on the phone, and she turned him down - speaking alternately as herself and as Eloise.

"Good night," Coppola said, finally hanging up, "to both of you."

Convinced in 1954 by an editor at Harper's Bazaar that her little-girl character could make a book, Thompson was put in touch with Knight, a magazine illustrator - and holed herself up for six months in the Plaza to write it.

"Why the Plaza?" Knight asks. "Because Kay had worked there and had liked the hotel."

It was a fortuitous choice. It was partly Eloise's association with the hotel that lent the book a real-life touch of New York sophistication - and has helped enshrine her, an ageless sprite running up the stairs.

Designed in 1907 by Henry J. Hardenbergh, the architect of the famous Dakota apartments a dozen blocks north, the Plaza sits at a corner of Central Park, a huge French chateau of a hotel. New York Times architecture critic Paul Goldberger calls it a city landmark more beloved than Grand Central Station. In the '50s and '60s, the hotel was tarted up under several owners, until it was bought by Donald Trump in 1988. Ivana Trump set about a series of controversial renovations, renovations that Knight sums up as "doing a lot of good, ripping out old, false ceilings, opening up things that had been covered up. And then putting too much gilt everywhere.

"But it's still an extraordinary building, still beautiful."

Partly because she is a solitary girl with a wealthy, absent mother, a persistent rumor about Eloise is that she is based on Liza Minnelli. Thompson was close to Judy Garland; she is Minnelli's godmother. In fact, these days, Thompson lives in Minnelli's New York apartment, and last year, she threw a birthday party for her - at the Plaza, naturally.

But although "Liza was about that age," Knight notes, "I think it was just a gossip column item that hung on. I've met women over the years who claimed to be the model for Eloise, and I've no doubt Kay met such children in hotels when she toured."

But, he argues, the source for Eloise is the most obvious one: Thompson herself. "I think she's drawn from her own consciousness. She's an aspect of Kay."

As for the model for how Eloise looks, Knight, 69, nods his head at a painting by his artist mother, hanging in his print-and-poster-stuffed apartment on New York's East Side. It's not Eloise; the young girl looks too serene in her black-velvet dress. But there is a similarity in style. She could be Eloise's older, more refined sister.

Knight cites a number of other influences, including artist Edmund Dulac, early 19th-century French songbooks illustrated by Maurice Boutet de Monvil and cartoonist Ronald Searle, whose own schoolgirl drawings led to the "St. Trinian's" British film comedies.

For those who know the Eloise books, one of the mysteries over the years has been why the sequels ("Eloise at Christmastime," "Eloise in Paris" and "Eloise in Moscow") are out of print. They've become rare items. Books of Wonder, a Manhattan store, recently sold a copy of "Eloise in Paris" for $350.

"Kay wants 'Eloise' to be perfect," Knight explains. "She says that it doesn't need the other books."

The sequels are indeed of lesser quality: In "Eloise at Christmastime," for instance, Eloise is turned into just another holiday-happy child. But the desire for uniqueness - for an original work that stands alone, complete, inviolate - extends to other Thompson projects, even somewhat to herself. Interviewers have cited her impeccable clothes sense, her flair for pronouncements, her perpetual-motion energy.

"No point in saving memorabilia," she told Reed in 1972. ``Somebody always steals it. I own an orange tree here, a rattan chair there, and the rest is in storage in Rome."

In "Funny Face" - the 1957 movie musical with Fred Astaire and Audrey Hepburn - Thompson makes a vivid, dryly comic impression as the Diana Vreeland-ish editor-dictator of a fashion magazine ("Think pink!").

"It is practically her only movie," Knight says, "certainly her only movie musical. Ms. Thompson also appeared with Minnelli in 1969's 'Tell Me That You Love Me, Junie Moon.' She didn't want any others. She did that one thing and that was it."

For his part, Knight has illustrated 60 other children's books and continues to work (he's drawn several famous Broadway posters, including "No, No Nanette" and "Sugar Babies"). But other than 1965's "Where's Wally?," a book about an escaped orangutan that was overshadowed by the highly similar "Where's Waldo?" books, nothing Knight has done has equaled his ``Eloise" achievement.

And that achievement has remained ... restricted to a single volume. Thompson's perfectionism, Knight explains, has extended to Eloise dolls and dresses and other spinoffs: No more are being produced.

"We've always been anxious for everything to be done in the best possible way," he says. And the dolls were not the best, although the clothes were, he maintains. In addition, Thompson apparently didn't like a "Playhouse 90" TV adaptation in 1956 (although she appeared in it with Ethel Barrymore). Ever since, she has turned down "thousands" of offers for animated cartoons, live-action versions, even a ballet.

These days, there is a rare, even courageous purity in such a stand: Compare what commercialization has done to Winnie-the-Pooh. But there is also a desire to freeze time, to keep her child her own.

"With all of the advances in computer technology these days," Knight says wistfully, "I think an animated version could be wonderful - if still absolutely kept under control. But..." he adds and slowly shrugs, leaving the obvious unsaid.

Thompson is nothing if not strong-willed. She hasn't even spoken to him, he says, in more than 10 years.

So the book and its author remain, unique, irrepressible, utterly themselves, this 82-year-old woman and her un-aging, 6-year-old shadow self.

Kay Thompson and me, ELOISE.

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