Over the next two decades, Geisel concocted such delightfully loopy tales as The 500 Hats of Bartholomew Cubbins and Horton Hears a Who. Most of his books earned excellent reviews, and three received Caldecott Honor Awards. But it was the 1957 publication of The Cat in the Hat that catapulted Geisel to celebrity.

Rudolf Flesch's book Why Johnny Can't Read, along with a related Life magazine article, had recently charged that children's primers were too pallid and bland to inspire an interest in reading. The Cat in the Hat, written with 220 words from a first-grade vocabulary list, "worked like a karate chop on the weary little world of Dick, Jane and Spot," as Ellen Goodman wrote in The Detroit Free Press. With its vivid illustrations, rhyming text and topsy-turvy plot, Geisel's book for beginning readers was anything but bland. It sold nearly a million copies within three years.

Geisel was named president of Beginner Books, a new venture of Random House, where he worked with writers and artists like P.D. Eastman, Michael Frith, Al Perkins, and Roy McKie, some of whom collaborated with him on book projects. For books he wrote but didn't illustrate, Geisel used the pen name Theo LeSieg (LeSieg is Geisel spelled backwards).

As Dr. Seuss, he continued to write bestsellers. Some, like Green Eggs and Ham and the tongue-twisting Fox in Socks, were aimed at beginning readers. Others could be read by older children or read aloud by parents, who were often as captivated as their kids by Geisel's wit and imagination. Geisel's visual style appealed to television and film directors, too: The animator Chuck Jones, who had worked with Geisel on a series of Army training films, brought How the Grinch Stole Christmas! to life as a hugely popular animated TV special in 1966. A live-action movie starring Jim Carrey as the Grinch was released in 2000.

Many Dr. Seuss stories have serious undertones: The Butter Battle Book, for example, parodies the nuclear arms race. But whether he was teaching vocabulary words or values, Geisel never wrote plodding lesson books. All his stories are animated by a lively sense of visual and verbal play. At the time of his death in 1991, his books had sold more than 200 million copies. Bennett Cerf, Geisel's publisher, liked to say that of all the distinguished authors he had worked with, only one was a genius: Dr. Seuss.