The Lacquer Screen
From Goodreads: "Early in his career, Judge Dee visits a senior magistrate who shows him a beautiful lacquer screen on which a scene of lovers has been mysteriously altered to show the man stabbing his lover. The magistrate fears he is losing his mind and will murder his own wife. Meanwhile, a banker has inexplicably killed himself, and a lovely lady has allowed Dee's lieutenant, Chiao Tai, to believe she is a courtesan. Dee and Chiao Tai go incognito among a gang of robbers to solve this mystery, and find the leader of the robbers is more honorable than the magistrate."