Wraiths and Changelings by Gladys Mitchell

An evening of exchanged ghost stories on a windy Hallowe’en night leads Dee Crieff-Tweedle to an inspired idea: she’ll organize a ghost hunt. She and her complacent husband (affectionately called and “Dum and “) soon organize a group of thrill seekers, including a curiously muscular clergyman, a male medium calling himself Madame Arcati, and Eiladh Gavin, Laura’s daughter and Dame B.’s goddaughter.Dee has planned out the itinerary with a thoroughness that would make a sergeant-major proud. Among the sites to investigate: a ghostly mass in a Salhouse churchyard, a body thrown over a Roman Fort wall in Burgh Castle, and a benevolent ghost monk (with dutiful ghost dog) in a ghost boat in the waters of Ranworth Broad. In Eiladh’s letters to Dame Beatrice, she reports that there’s trouble from the start, due to ill will between hostess and just about everyone else in the party. Eiladh records an escalating series of practical jokes to re-enact the ghostly scenarios, culminating in the strangling murders of two group members. Dame B. steps in and, after a round of interviews, clears Eiladh’s name whilst collaring the murderer(s?).