The Witch of the Low Tide;

Lady Betty Calder is a prostitute and a blackmailer - or is it her sister, Glynis, using her name? Dr David Garth, her fiance, knows he must find out the truth - especially when he blunders across Glynis' strangled body on Betty's property. The police know she did it, but David knows she didn't, and he must outwit a cunning murderer and a hostile detective-inspector to prove it. What he discovers - about his best friend's wife, his medical assistant, and even his fiancee - make him wish the blackmailing Glynis had never lived. 'John Dickson Carr's flair for the impossible crime shows through at its very best in this historical puzzler' The Times