The Boomerang Clue by Agatha Christie

After a vicar’s son discovers a dying man on a cliffside whose final words pose a baffling question, he teams up with a daring friend to investigate. Their search winds through country houses and hidden identities, as they survive poison and staged accidents and uncover a scheme of inheritance fraud and murder centered on a swapped photograph and a forged will. The cryptic question ultimately points to an overlooked witness whose testimony exposes the culprits.