By 1960—just three years after it was published—The Cat in the Hat had sold a million copies. Not bad for a trouble-making cat and a back-talking fish. (Source.)
Dr. Seuss was an artiste with a very cat-filled private collection. Check out Plethora of Cats for a taste or two hundred.
Seuss was quite the practical joker. He once convinced a friend that he was in possession of paintings by a famous Mexican artist—who didn't exist. Seuss worked on a painting overnight and sold it to his friend for 500 bucks. Pretty sneaky. (Source, 48.)
The Cat in the Hat was written from a list of just 220 words. Where did they come from? Check out "Writing Style" to find out.