Dion Fortune's The Winged Bull

A wise magician wants to free his sister from an obscene and dangerous ritual. He summons the ancient cult of the Winged Bull to infiltrate the Black Mass in which she faces the ordeal which will be literally worse than death. Magical practices and esoteric laws are described in dramatic form. Lover, The Sea Priestess, Moon Magic and The Secrets of Dr Taverner, as well as several non-fiction studies of the occult.