The Lacquer Screen by Robert Van Gulik

In 663, Judge Dee is the young magistrate in the fictional Chinese town of Peng-lai. On a visit to a senior magistrate Teng in Wei-ping, he is shown a beautiful lacquer screen which is mysteriously altered to show a murder scene instead of a love scene. With the senior magistrate Teng convinced he is going insane, a wealthy banker in town appears to kill himself, though it might be murder. Judge Dee and his servant Chiao Tai disguise themselves to go undercover and join a gang of robbers to solve the case.