I wasn't aware of this, but that's cool! A lot of them are a clash of cultures -- the North and Harriet Beecher Stowe produced Uncle Tom's Cabin, so the South produced Little Eve: The Flower of the South. Dr. Suess (his books are AMAZING for kids btw if you didn't already know) wrote Lorax defending the forests, and the logging industry wrote Truax in defense. And as much as I appreciated the themes and story behind The Chronicles of Narnia, I want to read the books written in response to them because I want to know both sides, you know? And then of course, is Go the F*** to Sleep, written in parody of the lovely sleepytime book It's Time to Sleep, My Love. XD
Really interesting stuff! There's more, too.
Go see!