Folly Du Jour by Barbara Cleverly
This is the seventh in the Joe Sandilands murder mystery series. It is set in Folies Bergere, Paris, December 1926. Joe hurries to the assistance of an old friend who has been arrested for murder. In a cell of the Quai des Orfevres he meets with Sir George Jardine, still in the evening dress stained with the blood of the dead man. The only other witness, a blonde who was sharing the victim's box, has vanished. Assistance for Joe comes from an entirely unexpected quarter - Francine, a young usherette, clawing her way into the world of the Paris Music Hall. It is she who becomes Joe's guide through this treacherous place where Joe is sure the killer is lurking.