Ethel Lina White 
Some Must Watch (British Mystery Classic) [EPUB ebook] 

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Set in early 20th century England, on the Welsh border, the novel tells the story of a serial killer who murders disabled young women in the community. His next victim apparently is Helen, a mute girl working as a maid for the wealthy, bedridden Mrs. Warren. Mrs. Warren urges her to leave the house, as does Dr. Parry, who knows the reason for Helen’s loss of speech and hopes to help her get her voice back.
Ethel Lina White (1876-1944) was a British crime writer, best known for her novel The Wheel Spins, on which the Alfred Hitchcock film, The Lady Vanishes, was based.