Death in Ecstasy (Dead Letter Mysteries)

Blurb

Death in Ecstasy is a detective novel by Ngaio Marsh; it is the fourth novel to feature Roderick Alleyn, and was first published in 1936.
When lovely Cara Quayne dropped dead to the floor after drinking the ritual wine at the House of the Sacred Flame, she was having a religious experience of a sort unsuspected by the other initiates. Discovering how the fatal prussic acid got into the bizarre group's wine is but one of the perplexing riddles that confronts Scotland Yard's Inspector Roderick Alleyn when he's called to discover who sent this wealthy cult member to her death.