He died of heart and kidney failure, said his brother, Stanley.
Mr. Asimov was amazingly prolific, writing nearly 500 books on a wide range of subjects, from works for preschoolers to college textbooks. He was perhaps best known for his science fiction and was a pioneer in elevating the genre from pulp-magazine adventure to a more intellectual level that dealt with sociology, history, mathematics and science. But he also wrote mysteries, as well as critically acclaimed books about the Bible, physics, chemistry, biology, astronomy, limericks, humor, Shakespeare, Gilbert and Sullivan, ancient and modern history, and many other subjects.
Mr. Asimov's first book, "Pebble in the Sky" (Ballantine), a science-fiction novel, was published in 1950. His first 100 books took him 237 months, or almost 20 years, until October 1969, to write. His second 100, a milestone he reached in March 1979, took 113 months, or about 9 1/2 years -- a rate of more than 10 books a year. His third 100 took only 69 months, until December 1984, or less than 6 years.
"Writing is more fun than ever," he said in a 1984 interview. "The longer I write, the easier it gets."
He once explained how he came to write "Asimov's Guide to Shakespeare" (Crown). It began, he said, with a book called "Words of Science." " 'Science' led to 'Words on the Map,' " he remarked, "which took me to 'The Greeks,' which led me to 'The Roman Republic,' 'The Roman Empire,' 'The Egyptians,' 'The Near East,' 'The Dark Ages,' 'The Shaping of England' and then 'Words From History.' It was an easy jump to 'Words in Genesis,' which brought on 'Words From the Exodus.' That led me to 'Asimov's Guide to the Old Testament,' and then 'The New Testament.' So what was left except Shakespeare?"