A. A. (Alan Alexander) Milne 
Red House Mystery 

The setting is an eng 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. Mark’s 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… (Excerpt from Wikipedia)