The Gnome King of Oz is the 21st book in the Oz series, continuing the stories originally created by L. Frank Baum. However, "The Gnome King of Oz" is not written by L. Frank Baum, who wrote the first 14 Oz books, but by Ruth Plumly Thompson. Thompson took over the Oz series after Baum's death, and she wrote a total of 19 Oz books. Ruth Plumly Thompson's contributions to the Oz series extended the magical universe created by L. Frank Baum. While not as widely recognized as Baum, her Oz books have been appreciated by fans of the series.