First published in 1991 by Necronomicon Press as a Hardback 

Witches of the Mind is a complex and interesting study of Fritz, his life and his writings, unfortunately it is expensive and difficult to find, luckily I got to read a friends copy…  with regards to the content, Bruce Byfield’s blog contains copious information on it’s genesis.
Bruce Byfield’s notes on the book

Review at Swords and Sorcery