Every afternoon, New York City's Plaza Hotel serves high tea in the Palm Court, a bright winter garden with towering palms and a harpist. And every day, among the set of silver-haired dames in wool suits are demure but excited little girls who have come to pay homage to Eloise. They nibble on cucumber sandwiches and rocky scones, just like their beloved literary heroine.

"They think she's real," says Eloise's illustrator, Hilary Knight, 75, who first drew the plucky character in the 1955 book Kay Thompson's Eloise. In the next decade, he and the eccentric Thompson produced four more books featuring the 6-year-old imp who resides at the Plaza with her saucy au pair, Nanny, and who thrives on creating elevator traffic jams.

On Oct. 22, almost 50 years after her debut, that naughty little girl returns in Eloise Takes a Bawth, the series' fourth tale, written by Thompson in the 1960s but never published. It is the first "new" Eloise book in 40 years. Fans, aware of its existence, have long clamored for it, and Simon & Schuster is so certain of the pent-up demand that it is printing 200,000 copies.

Devoid of all prissiness and manners, Eloise was the original antidote to the girl next door. The first Eloise book proved a startling publishing success, sharing the best-seller list with works by Graham Greene and John O'Hara. To date, it has sold more than 2 million copies. But the three other sequels — Eloise in Paris, Eloise at Christmastime and Eloise in Moscow — were yanked out of print by Thompson in the mid-'60s. She let the original remain. After Thompson's death in 1998, her estate allowed Simon & Schuster to resurrect the three sequels the next year, and together they have since sold more than half a million copies. Eloise Takes a Bawth had made it as far as the printing press when Thompson pulled it back. The only reason she ever gave, says Knight, was that she did not think it was as "perfect" as their first book.

Unlike the other Eloise books, Bawth is not simply a precocious child's antics and musings. "Bawth has something of a story line," says Knight. "It's an event that takes place and is carried through an entire day." As Eloise lies in the tub, we get a tour of her aquatic fantasies — captaining a pirate ship, water-skiing with her pet turtle — while she accidentally floods the entire hotel on the night of the lavish Venetian Masked Ball.

The book features two signature Knight gatefold illustrations that are so rich in visual jokes and details, they can be studied for hours. "He knows how to imbue characters with emotional characterizations," says legendary writer and artist Maurice Sendak. "He knows how to punctuate and fill everything with graphic prowess."

Bawth's release is rewarding Knight with some late-career fanfare. During the initial Eloise craze, he was often overshadowed by the zany Thompson, an accomplished nightclub performer and voice coach to such stars as Judy Garland and Lena Horne. Given to bouts of melodrama, she once sawed the legs off her baby grand piano so that she could serenade her pug "eyeball to eyeball." By all accounts, her sanity teetered as she aged. She spent her last years holed up in the apartment of her goddaughter Liza Minnelli, refusing contact with almost everyone.

When Thompson began to withdraw Eloise from the public — some say she did not want to compete with her fictional creation's fame — Knight pursued other projects. He went on to illustrate more than 60 books (Where's Wallace?, Sunday Morning). He is currently a staff artist for Vanity Fair. But it's his iconic depiction of Eloise, the enfant terrible with porcupine-needle hair, that he will be known for.

The Palm Court does not take reservations for afternoon tea, but when a reporter mentions that her guest will be Mr. Hilary Knight, she is promptly booked. At tea, a waiter brings complimentary glasses of champagne. Knight, a natty, exceedingly polite gentleman in a black silk shirt, still adheres to a rigorous work schedule in his Manhattan home studio. He recalls being summoned to Rome in 1963 to work with Thompson on Bawth: "Unlike the other books, which took a year each, this went on for four years." When Bawth was given its rebirth, no one was more excited than Knight. "Inasmuch as it took, really, 40 years to get this going," he says, "it's wonderful to have it finally finished."

• Heather Won Tesoriero