Stanley Weinbaum 
The Black Flame [EPUB ebook] 

Civilization had died screaming in a blaze of nuclear and biological warfare. Gradually, during the Dark Centuries that followed, secrets were re-discovered, and new knowledge gained. Finally, the key to immortality is found, and the preternaturally beautiful Margaret Smith becomes Margot of Urbs and, with her brother, begins a relentless drive to absolute power…