Phantom Hollow — Gerald Verner

When Tony Frost and his colleague Jack Denton arrive for a holiday at Monk's Lodge, an ancient cottage deep in the Somerset countryside, they are immediately warned off by the local villagers and a message scrawled in crimson across a windowpane: 'THERE IS DANGER. GO WHILE YOU CAN!' Tony invites his friend, the famous dramatist and criminologist Trevor Lowe, to come and help - but the investigation takes a sinister turn when the dead body of a missing estate agent is found behind a locked door in the cottage...