This book is recommended by Pythia
https://www.amazon.com/gp/product/1567184006/ref=ox_sc_sfl_title_65?ie=UTF8&psc=1&smid=ATVPDKIKX0DER
Read it!
I know for sure, Pythia's knowledge of astrology definitely came from her study of the old masters, like Robson, Carter, Volguine and the like, and her own research of course
Most of her astrological interpretations were taken from Frances Sakoian's "The Astrologers Handbook"
If you'd walk into her house, you'd probably fine thousands of books, and I'm not even exaggerating