Maud Silver -- a Miss Marple of a well-brought spinster, who is in the private inquiry business. Written in the 50's but set a little before that. This is the fourth in the series.
In this mystery, Miss Silver needs to find out if Lady Dryden murdered Sir Herbert. She has been found standing over his body with a dagger.
From the back of the book:
Immediately on her arrival at Ladies' House, Ione Muir realizes that all is not right with her newly married sister, Allegra. The town of Bleake is charming, but the medieval manor hourse itself-known by a sinister local legend as Ladies' Bane-is full of gloomy depths and twisting staircases. Her brother-in-law Geoffrey Trent, intends to purchase the expensive property, with which he is clearly infatuated, although his wife seems frightened by it.
Miss Silver, hired by Allegra's godmother to investigate Geoffrey's finances, is the first to suspect foul play when his mischevious young ward, Margot, dies in a accident. Her keen intellect and uncanny understanding of human nature help Miss Silver to reveal the ominus forces that are at work in this strange household.