Now I know why Georgette Heyer's books still circulate so well in the library system. This was a classic country house who done it, complete with an atmospheric old mansion, quirky characters, and a quiet professional from Scotland Yard. What made it great was the plot where everyone could have done it and the dialogue, which sometimes had me laughing out loud. They Found Him Dead features the Kane family, gathered at Cliff House to celebrate Silas Kane's 60th birthday. The celebration turns to shock when Silas is found dead at the bottom of the cliff the next day, but surely it was an accident. Then another family member is struck down and it becomes clear that a murderer is in their midst. But who could it be and who might be next? If you've never read Heyer I suggest you give her a try. I listened to the audio narrated by Clifford Norgate. The narration added to the story as Norgate's interpretation was spot on with accents and timing. An older book on cassette, but if you can find it, it's worth a listen.
Heyer wrote several mystery novels, but she is most well known for the her regency romances, many of which are available as eBooks, downloadable for free from the library.
If you're looking for a mystery of similar time and feel reach for a classic Agatha Christie or Dorothy L. Sayers.
If you'd like to read about this prolific and lasting author, try Georgette Heyer by Jennifer Kloester.