The Secret Adversary by Agatha Christie

Classic mystery novel. According to Wikipedia: Dame Agatha Christie, DBE, (15 September 1890 – 12 January 1976), was a British crime writer of novels, short stories and plays. She also wrote romances under the name Mary Westmacott, but she is best remembered for her 80 detective novels—especially those featuring Hercule Poirot and Miss Jane Marple—and her successful West End theatre plays. According to the Guinness Book of World Records, Christie is the best-selling writer of books of all time and, with William Shakespeare, the best-selling author of any kind. Only the Bible has sold more than her roughly four billion copies of novels.According to UNESCO, Christie is the most translated individual author, with only the collective corporate works of Walt Disney Productions surpassing her. Her books have been translated into at least 103 languages.