The Genocides, Thomas M. Disch (1965)
An unflinching exploration of human behavior in an apocalyptic context. Disch's central characters strain to rigidly persist in the patterns of their pre-apocalypse lives, or fall apart, or evolve. The writing is sharp and unpretentious. I can imagine this as a film from my parents' youth that ends up on late-night TV, that they reluctantly allow me to stay up late watching; or as a world visited by the Starship Enterprise. Thematically brutal. Likely to satisfy SF fans and, from a craft standpoint, worth reading twice.