Not to Disturb. Muriel Spark

Behind the high walls of a mansion in Geneva a night of sinister revelry is about to begin . . .
In the staff quarters, the servants led by the cool, unflappable butler are preparing for the downfall of the Baron and Baroness. Meanwhile in the attic, the Baron's invalid brother awaits his fate as an unwitting pawn in their devious plans. And in the library, the Baron, the Baroness and their young handsome secretary are locked in a mysterious, heated discussion.
As the macabre scenario plays itself out, a world of grim humour and gruesome possibilities unfolds . . .