Agatha Christie 
The Man in the Brown Suit 

Anne Beddingfeld is always ready for an adventure. So when she witnesses a man wearing a brown suit die at a tube station in London, she searches for clues and finds a mysterious piece of paper nearby. The message it contains leads her on a confounding chase full of secret aliases and codes as she seeks to solve the case and catch the murderer. Featuring an appearance from Secret Service agent Colonel Race, this story from renowned mystery writer Agatha Christie is a true classic that blends adventure and suspense with the detective story.