Intruder in the Dark — George Bellairs

A corpse in a country house brings Scotland Yard to an eerily quiet English village, in this tale by a master of British mystery.
 
Cyril Savage has inherited the home of his wealthy and estranged aunt. But before Savage has the chance to discover her fortune, he is struck dead in the cellar of this once grand country house in the strange, nearly deserted village of Plumpton Bois.
 
The police are baffled and -- unable to unearth a motive, let alone a killer -- call for the assistance of Scotland Yard. Inspector Littlejohn and Inspector Cromwell arrive in Plumpton Bois and find the village, the family, and the house itself full of secrets.
 
The door to a locked room has been bashed open. Savage's aunt is not nearly as rich as she seemed to be. And now, another body has turned up on Littlejohn's watch . . .