I didn't mention The Purple Cloud before, because it is 20th century (by two years, 1901) but it's a great read. It influenced Stephen King with The Stand and Lovecraft praises it in his "Supernatural Horror" essay. A cyanide gas cloud kills everything on the planet except for one man who was at the North Pole at the time. He comes back and finds himself the ruler of the entire world... so what does he do with it?

I picked up an edition from the 30s that is about to fall apart, and I got a nice edition from Tartarus Press a while back.

A perfect Monster has no end...