The Case of the Registered Letter

Grace Isabel Colbron (1869-1943) was an American author of detective fiction. Writing career, especially in such genre, was not common for women in that time. Her writings can be described as belonging to detective subgenre of "Whodunit". Her most notable work is a series of novels, united by the single protagonist, Joe Muller, and called A Case Collection.
The Case of the Registered Letter is a detective novel by written by Augusta Groner and translated by Grace Isabel Colbron who included this story into hers Case Collection. Like other novels of this series, this one follows Joe Muller, a hardworking Austrian policeman, who once again has to dig deep and think wide to solve a mysterious murder.