Green Eggs and Ham was a fifty-dollar bet
His publisher, Bennett Cerf, wagered fifty dollars that he could not write a book using fifty different words or fewer. The result uses exactly fifty: SAM, I, am, that, do, not, like, GREEN, EGGS and HAM among them, with only one word of more than one syllable — anywhere.
It became one of the best-selling children's books ever written. Seuss said afterwards that Cerf never paid up. The constraint is the whole reason it works: with fifty words to play with, the only tools left are rhythm and repetition, which happen to be exactly what a child learning to read can hold on to.
The Cat in the Hat was written to fix a problem
In 1954 a magazine article argued that children were not learning to read because the books schools gave them — Dick and Jane and their unbelievably tedious dog — were so boring that no child would choose one. A publisher agreed, sent Seuss a list of about 350 words a six-year-old should know, and asked him to make something out of it.
He used 236 of them, and later said the title came from the first two words on the list that rhymed. The book sold a million copies within three years and effectively ended the Dick and Jane era. It is a rare case of a genuine educational problem being solved by making the thing more fun rather than more worthy.
He was not a doctor, and it is not pronounced Seuss
Theodor Geisel added the Dr while at Oxford, partly as a joke about the doctorate his father wanted him to get and never finished. SEUSS was his middle name and his mother's maiden name, and it is German — properly said to rhyme with voice, not with juice.
He gave up correcting people early on, reportedly deciding the American pronunciation was useful because it rhymed with Mother Goose. His first book was rejected somewhere between twenty and forty times depending on who is telling it, and he was walking home to burn the manuscript when he met an old college friend who had that week become a children's editor.
The books are usually arguing about something
Under the invented animals there is almost always a point. The SNEETCHES, with and without stars on their bellies, is about prejudice and was written in response to antisemitism. YERTLE the turtle, stacking his subjects to sit higher, was written about dictatorship. The ZAX are two creatures who refuse to step aside for one another and stand there while a motorway is built over them.
The LORAX is the most direct: TRUFFULA trees cut down for profit until the last one goes. HORTON hears a WHO and insists a person's a person no matter how small. Then there is the pure nonsense — the CAT, the HAT, THING One and Two, ONEFISH two fish, the WHOS of WHOVILLE, the GRINCH and CINDY Lou, and the street called MULBERRY where the RHYME began.