They Never Say When is a 1944 thriller novel by the British writer Peter Cheyney.Reilly p.300 It is the sixth in his series of novels featuring the London private detective Slim Callaghan, a British version of the increasingly popular hardboiled American detectives.
Synopsis Callaghan is hired by a Mrs Paula Denys who claims that she paid a to steal a priceless coronet from her husband's safe. Now he refuses to hand over the stolen and is instead trying to blackmail her.
References
Bibliography Magill, Frank Northen. Critical Survey of Mystery and Detective Fiction: Authors, Volume 1. Salem Press, 1988. Reilly, John M. Twentieth Century Crime & Mystery Writers. Springer, 2015. Server, Lee. Encyclopedia of Pulp Fiction Writers. Infobase Publishing, 2014.