Turns out "Shrek", the beloved movie of children and manbabies everwhere, was based on a book.
No one probably cares, but Shrek was probably my favorite movie when I was 10-ish - I could quote the movie in its entirety at one point (I'm down to probably 60% now). I always figured it was an original IP. This completely blew my mind in a bookstore. It was made in 1990.
That's it. Steig totally fucking sold out, letting them make Shrek 3 and all those awful tie-in games.