Margaret St Clair 
Sign of the Labrys [EPUB ebook] 

Earth was a weird and dire place after the plagues. The few humans who survived could not bear the touch of each other; they lived in the enormous, endless caverns hacked out of the bowels of the earth for the bombs that never came.And on one man rested the hopes of the world, though he did not know it. Sam Sewell only knew he had to journey, despite forbidding perils from the darkness of the past, into the ultimate fastnesses of the unknown to rescue the timeless wisdom of the witch Desponia . . .