The novel is a combination of mystery and spy story, with some romance added in on top of the action. There are secret passages to be explored, lots of government and villainous intrigue, secret formulas, and strange people coming and going. (Goodreads)
Dora Amy Elles (15 October 1877 – 28 January 1961[2]), who wrote as Patricia Wentworth, was a British crime fiction writer. Wentworth wrote a series of 32 crime novels in the classic whodunit style, featuring Miss Maud Silver, a retired governess and teacher who becomes a professional private detective, in London, England. Miss Silver works closely with Scotland Yard, especially Inspector Frank Abbott, and is fond of quoting the poet Tennyson. Miss Silver is sometimes compared to Jane Marple, the elderly detective created by Agatha Christie.