The cause was a stroke, his daughter Peggy Adler said.
Mr. Adler joined the American Communist Party in 1935, when he was 22. Sixteen years later, when he was chairman of the math department at Straubenmuller Textile High School on West 18th Street in Manhattan, he was subpoenaed to testify before a Senate subcommittee investigating Communist influence in the nation's schools. He refused to answer the senators' questions, citing his rights under the Fifth Amendment.
Within weeks, he was taken away from his students.
"I was teaching a class when the principal sent up a letter he had just received from the superintendent announcing my suspension, as of the close of day," he recalled in 2009. He was later dismissed.
Mr. Adler was among more than 1,150 teachers who, in the anti-Communist furor of the cold war, were investigated under New York State's Feinberg Law. Enacted in 1949, the law directed the Board of Regents to list organizations it considered subversive and deemed membership in those organizations prima facie evidence for firing any public school employee.
Called into the office of the school superintendent, William Jansen, Mr. Adler was asked, "Are you now or have you ever been a member of the Communist Party?" Once again, he refused to answer. He was one of 378 city teachers ousted under the Feinberg Law and, based on his last name, became the lead plaintiff in the case known as Adler v. Board of Education.
In March 1952, after the case rose rapidly through the lower courts, the United States Supreme Court held in a 6-to-3 decision that there was "no constitutional infirmity" in the Feinberg Law, as Associate Justice Sherman Minton wrote in his opinion. Associate Justices William O. Douglas, Hugo Black and Felix Frankfurter dissented, declaring that the law "turns the school system into a spying project."
The decision stood until 1967, when, in Keyishian v. Board of Regents, the Supreme Court reversed it with a 5-to-4 ruling that the Feinberg Law and similar statutes were unconstitutional. Dozens of dismissed teachers were eventually reinstated, and in 1977 Mr. Adler began receiving his annual pension of $14,901.
In 1953, during a hearing on the charges against him, Mr. Adler had said, "If you asked me whether I was a Democrat or a Republican, I would tell you it was none of your business."
By then he had begun educating children in another way. His first book, "The Secret of Light," was published in 1952. Its opening sentences are something of a mission statement: "This is a book about light. It will tell you interesting facts about many simple, ordinary things, like a glass of water, mirrors, soap bubbles and hot pavement."
"Part of this story sounds like a fairy tale," it continues. "But the wonders it describes are all true. This does not make the story any less exciting, for there is no adventure more thrilling than discovering the real wonders of the world we live in."
The wonders that Mr. Adler would illuminate in his 87 books many written with and illustrated by his wife, Ruth Relis Adler are evident in their titles, among them "How Life Began," "The Stars: Steppingstones Into Space" and "Thinking Machines." In "Why? A Book of Reasons" (1961), he answered simple questions like "Why don't the fish freeze when the pond freezes?" and "Why can a fly walk on the ceiling?"
Mr. Adler's books, many of them written after he moved to Vermont, have sold over four million copies and been translated into 19 languages. He received awards from the Children's Book Council and the National Science Teachers Association.
Born in Manhattan on April 27, 1913, Irving Adler was one of five children of Marcus and Celia Adler, immigrants from what is now Poland. His father sold ice, coal, wood, seltzer and beer. Irving was an outstanding student, entering Townsend Harris Hall (now Townsend Harris High School) at 11 and graduating from City College with a degree in mathematics at 18. Soon afterward "he was teaching high school students that were older than him," his daughter said.
Mr. Adler married Ruth Relis in 1935; she died in 1968. His second wife, the former Joyce Lifshutz Sparer, died in 1999. Besides his daughter Peggy, he is survived by his son, Stephen; a stepdaughter, Laura Wallace; nine grandchildren; and nine great-grandchildren.
Not all of Mr. Adler's writing was directed at youngsters. In "Mathematics and Mental Growth" (1968), he reprised essays he had written for professional journals in support of the "new math" the movement to replace rote learning of the subject with clear delineation of mathematical concepts.
In reviewing the book for The New York Times, Isaac Asimov wrote, "It is to be hoped that interested parents will read them to see what one enlightened educator thinks."
"We race toward catastrophe by facing the Nuclear Age with our Tom Sawyerish idyll of education," Mr. Asimov added. "Adler's book is a cry for something better."