Here's a bit from seuss.org about how he got started, which I had never known:
In May of 1954, Life published a report concerning illiteracy among school children. The report said, among other things, that children were having trouble to read because their books were boring. This inspired Geisel's publisher, and prompted him to send Geisel a list of 400 words he felt were important, asked him to cut the list to 250 words (the publishers idea of how many words at one time a first grader could absorb), and write a book. Nine months later, Geisel, using 220 of the words given to him published The Cat in the Hat, which went on to instant success.
And from there, well.