The Red House Mystery by A.A. Milne

The setting is an English country house, where Mark Ablett has been entertaining a house party consisting of a widow and her marriageable daughter, a retired major, a wilful actress, and Bill Beverley, a young man about town. Marks long-lost brother Robert, the black sheep of the family, arrives from Australia and shortly thereafter is found dead, shot through the head. Mark Ablett has disappeared, so Tony Gillingham, a stranger who has just arrived to call on his friend Bill, decides to investigate. Gillingham plays Sherlock Holmes to his younger counterparts Doctor Watson; they progress almost playfully through the novel while the clues mount up and the theories abound.