The Witch of the Low Tide

A man fears his beloved is not who she claims to be in this sophisticated puzzler by John Dickson Carr, a master of the British-style detective novel
David Garth has just stepped off the train at Charing Cross when he is summoned to Scotland Yard to answer questions about a person believed to be living a double life.
A neurologist, Garth is an expert on the brain, but in matters of the heart he is clueless. He has fallen in love with Betty Calder, a delicate young woman whom the police suspect of blackmail and prostitution.
Garth refuses to believe these accusations, but when a strangled body is found on Betty's property, surrounded by fifty feet of wet sand with no footprints but her own, the challenge before him seems daunting. Can he outwit a cunning murderer and a hostile detective-inspector to prove his fiancee's innocence?