He died in 1973, and when the landlord went in to clean out Henry's apartment it was discovered that he had created massive books containing thousands of paintings of a strange, beautiful, fantasy world unlike anything anyone had ever seen.
Art experts were called in to deal with the situation, and the rest is history. Henry Darger's works are now known world wide. He was a Chicagoan (lived in Lincoln Park), and I happen to know one of the experts who was called in when the books were first discovered. She says that some of them were so large they had to take them apart to get them out of the tiny one-room apartment.
Also, here is a wonderful documentary on Henry Darger and his work*, I highly recommend it if you have any interest in learning more about this amazing artist.
Additional information on him is also here.
*For those of you outside the U.S who want to watch this I recommend using Hotspot Shield to hide your IP (it's free).