No discussion of Outside Art would be complete without mentioning Henry Darger (https://en.wikipedia.org/wiki/Henry_Darger) and his uber-epic In the Realms of the Unreal, a "15,145-page, single-spaced fantasy manuscript". He lived almost all his life on 851 W. Webster Avenue in Chicago. If you're in Chicago you can also visit the INtuit Center of Outsider Art (https://en.wikipedia.org/wiki/Intuit:_The_Center_for_Intuiti...) to see his work, in NYC the American Folk Art Museum has a center dedicated to him.

Or you can watch the documentary https://www.youtube.com/watch?v=sRlvDKcDvsI, although I didn't find it do too much justice to him or his work.