Josephine Saxton’s 1969 The Hieros Gamos of Sam and An Smith is a stand-alone post-apocalyptic novel.
The boy has wandered the world for a decade. It is a world well-supplied with fully-stocked stores and yet very nearly empty of animals and other people. Sensing that other people are dangerous, he seeks solitude.
This changes the day he finds a still-living newborn baby girl next to her mother’s corpse.