Earth Abides by George R. Stewart. Seminal post-apocolyptic novel. I'm about half way through; it's a bit of a slow starter but interesting nonetheless.
I also learned from this book (just yesterday) that apparently, in the first half of the 20th century, people believed that you could identify 'people of color' who were 'passing as white' by checking for a bluish tinge to the half-moons of the fingernails. Weird and not at all true, btw (as far as I could tell from the googling).