The Emperor's Pearl by Robert Van Gulik

Judge Dee, magistrate of Poo-yang a flourishing walled city on the Grand Canal, is attending the Dragon Boat races accompanied by his three ladies aboard his own official barge. He is mildly annoyed by the intrusions of assorted callers and the loss of a blank domino (he and his ladies are keen players). He is more than annoyed when the sad, sudden death of a young student crewing one of the boats turns out to be deliberate murder.

Even more disturbing is the murder of the young Second Lady of a prominent local merchant and collector which is witnessed by the Judge himself. Obviously very odd things are going on at the deserted villa at the edge of the River Goddess's overgrown mandrake grove! Throw in an apparently cursed Imperial Treasure and a perverted madman and the Judge has his hands full.