Marjorie Weinman Sharmat, who helped children discover the joys of reading with more than two dozen books in her Nate the Great series about a pancake-eating boy detective, died March 12 at a hospital in Munster, Ind. She was 90.

The cause was respiratory failure, said her son Andrew Sharmat.

Mrs. Sharmat wrote or co-wrote more than 130 books for young readers, her audiences ranging from children just learning to sound out words to adolescents peering into the grown-up world. Of all her characters, the best known was Nate the Great, who debuted in 1972 and became a favorite of the preschool and early-elementary set.

Her books about Nate, who was named after her father, sold 15 million copies, according to the publishing company Penguin Random House. Many of those volumes featured pictures by the Caldecott Medal-winning illustrator Marc Simont. He often depicted Mrs. Sharmat’s hero clad in a trench coat and deerstalker, a look of concentration on his face, his trusty dog Sludge by his side.

Nate the Great was, in the description of one Washington Post reviewer, the “sharpest young detective since Encyclopedia Brown graduated to the fifth grade,” a reference to the protagonist of Donald J. Sobol’s mystery books for slightly older readers. Mrs. Sharmat — who once described herself to the Arizona Daily Star as “very inquisitive” as a girl, “not nosy” but always looking for the “red herring” — found an audience among readers not yet old enough for the Hardy Boys and Nancy Drew, but not too young to follow a trail of clues.

Detective work is not fun and games,” Nate reports in “Nate the Great Goes Undercover” (1974) in which he holds a stakeout from inside a garbage can. “. . . Detective work is banana peels, dishrags, milk cartons, floor sweepings, cigar ashes, fleas and me all together in one can.”

Nate’s most salient quality, besides his talent for solving mysteries, was his taste for pancakes. Mrs. Sharmat chronicled his gumshoeing in titles including “Nate the Great and the Lost List” (1975), “Nate the Great and the Missing Key” (1981), “Nate the Great and the Fishy Prize” (1985), “Nate the Great and the Musical Note” (1990), “Nate the Great, San Francisco Detective” (2000) and “Nate the Great, Where Are You?” (2014).

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Reviewing Mrs. Sharmat’s work for The Post, book critic Michael Dirda noted the plainness of her prose aimed at children just learning to read. “One yearns for a subordinate clause, a compound-complex sentence being too much to hope for,” he observed. “And yet, for a year or two, these books will hold all the pleasure of Proust, the excitement of le Carre. Probably more. Parents may find such books a yawn, but their first-graders won’t.”

So great became Nate’s cultural penetrance that he was featured in “Got milk?” advertisements by the dairy industry and on Cheerios cereal boxes for a literacy campaign.

Mrs. Sharmat collaborated on her books over the years with her husband, Mitchell Sharmat, who was the author of “Gregory, the Terrible Eater” (1980); her sons, Andrew and Craig Sharmat; and her sister, Rosalind Weinman. With her husband, she wrote several books featuring Olivia Sharp, Nate the Great’s cousin and a detective in her own right.

“There are many installments in the Nate the Great mysteries,” Dirda wrote in his review — enough to sustain young readers not only until the age when they can read Encyclopedia Brown or Hardy Boys, he quipped, but until they can tackle Umberto Eco’s medieval murder mystery “The Name of the Rose.”

Marjorie Evelyn Weinman was born in Portland, Maine, on Nov. 12, 1928. Her father operated a dry goods store, and her mother was a homemaker.

Mrs. Sharmat, who grew up loving mysteries, told an interviewer that her “earliest ambition was to become a writer or a detective or a lion tamer.” She combined the first two interests by collaborating with a friend on a newsletter, the Snooper’s Gazette, their reportage consisting principally of listening in on adult conversations.

She worked in merchandising in New York City before moving to New Haven, Conn., where she worked at the libraries at Yale University and Yale Law School. Her first published literary work, written during that time, was a short story for adults.

“Most writers have an almost relentless desire to communicate,” she wrote in a commentary published in Cricket magazine. “It is perhaps like a sore tooth or a nagging headache, conspicuously unremittingly there. I have felt this compulsion ever since I can remember.”

Motherhood sparked her interest in children’s literature, her son Andrew said. She found many early-reader books painfully dull and set out to produce more amusing tales. Her first children’s book was “Rex,” published in 1967, about a boy who runs away and looks for a new home by pretending to be his neighbor’s dog.

Other picture books by Mrs. Sharmat included “Gila Monsters Meet You at the Airport” (1980), which she wrote when her family moved to Arizona from the East Coast, and “I Don’t Care” (1977), a book about feelings that features a boy who says he doesn’t mind — but really does — when his balloon floats away.

For adolescents, she wrote novels including “How to Meet a Gorgeous Guy” (1983) and “How to Meet a Gorgeous Girl” (1984), which counseled readers against following the kind of advice they might find in books with such titles.

Mrs. Sharmat moved to Indiana from Tucson in 2011. Her husband died that year after 54 years of marriage. Besides her son Andrew, of Munster, and her son Craig, of Santa Barbara, Calif., survivors include two grandchildren.

The adventures of Mrs. Sharmat’s sleuth were last chronicled in “Nate the Great and the Wandering Word,” co-written with Andrew Sharmat and featuring illustrations by Jody Wheeler, and published last year.