Spinsters in Jeopardy by Ngaio Marsh

Chief Detective Inspector Roderick Alleyn is on his way to Roqueville, France to do a bit of undercover detecting. He is also combining business with pleasure, and has brought his wife, Troy, and his five year old son Ricky to this beautiful part of France. Troy also plans while they are there to visit her cousin M. P.E. Garbel whom she does not remember, but who has been writing rather peculiar letters to her.

While they are still on the train to Roqueville, Troy and Alleyn see a very strange event through a window of a chateau by the side of the train track. The conductor tells them that this is the Chateau de la Chevre d'Argent This the place which Alleyn has come to investigate. On the morning on which they are to arrive at Roqueville, Miss Truebody, one of the passengers, has an attack of apendicitis. She is taken from the train. Alleyn finds that the town doctor is away at a medical conference, and that the only doctor nearby is Dr. Baradi who is a resident of the mysterious Chateau of la Chevre d'Argent. He hires a car driven by Raul Milano to take Miss Truebody, and the Alleyns to the Chateau.

Dr. Baradi is a rather suspicious looking Egyptian, but  appears to be a very competent physician. He operates on Miss Truebody in a makeshift operating room with the assistance of Alleyn and Raul. Meanwhile, Troy and Ricky meet some of the other occupants of the Chateau. There is Mr. Oberon who owns the chateau, an actress, an artist, and a young couple. All in all,  this seems to be a place offering spiritual advice to the well-to-do, and the famous.

The story becomes more complex and many questions may be asked.  What are the religious rites at the Chateau? What is actually being done there? Will Alleyn's undercover identity be revealed and his true occupation become know? Where does Ricky go? Who is Troy's mysterious cousin? What did Troy and Alleyn see from the train window? Who are the spinsters who are in jeopary?

This book was first published in 1953. Used copies are available at Amazon. A 1960 printing has the title "Spinsters in Jeopardy: A Fresh Victim for the Black Mass" which gives you a clue about what is going on.