The Black Cabinet, by Patricia Wentworth (epub/Kindle)
A poor but beautiful young girl is made heiress of her mysterious cousin who has made a tidy fortune in blackmailing. The capital—indiscreet letters which would rule the lives of all sorts of distinguished English people—is kept secret in a safe behind a black cabinet. Cousin Dane whispers the combination to Chloe and straightaway dies. His mistake in his career was in having accomplices and poor Chloe falls into their hands. They stop at nothing, but she succeeds in opening the safe and destroying the letters.