Penhallow, by Georgette Heyer
Heyer's mysteries are hit-and-miss for me. Some of them aren't any good at all, and some are fine. This one is quite good, but darker than Heyer's usual tone. It's really not quite a mystery; more of a novel with a mystery in it, because readers know perfectly well who did it, but the characters do not.
Penhallow is both the Cornish estate and the owner. Old Penhallow is a tyrant who rules the house from his bedroom and insists on having all of his grown children there, under his eye, no matter what they would rather be doing. Although he's always been domineering and insensitive, he's getting worse and is now just plain gleefully cruel. But when one family member thinks to relieve the intolerable situation by hastening his death, it becomes ten times worse.
It's a dark and tragic story, but gripping and hard to put down. One of Heyer's better mysteries, I should say.