https://www.amazon.com/Sundown-Towns-Hidden-Dimension-Americ...
My hazy understanding of American racism, which I now know to be wrong, was that it's always been getting better. Sure, slavery, but then the Civil War and an upward climb from there to the Civil Rights era and beyond. Except for the South, I thought.
It turns out the US had period after Reconstruction, know as the Nadir [1], when anti-black sentiment and action grew significantly. A wave of ethnic cleansing circa 1890-1920 led to a lot of all-white towns all over the country, ones where non-whites weren't allowed after dark. (Thus the title of the book.)
Chapter 11, "The Effect of Sundown Towns on Whites", talks a lot about how growing up in white-only areas leads white people to have enormously distorted perceptions of the dangers of black people. After reading that chapter, the RacistDoor phenomenon made a lot more sense to me.
[1] https://www.google.com/search?client=ubuntu&channel=fs&q=the...