Ngaio Marsh 
Spinsters in Jeopardy [EPUB ebook] 

A classic Ngaio Marsh mystery thriller combining drugs and sacrifice.High in mountains stands the magnificent Saracen fortress, home of the mysterious Mr Oberon, leader of a coven of witches. It is not the historic castle, however, that intrigues Roderick Alleyn, on holiday with his family, but the suspicion that a huge drugs ring operates from within its ancient portals.But before the holiday is over, someone else has stumbled upon the secret. And Mr Oberon decides his strange and terrible rituals require a human sacrifice…