The Lost Princess of Oz (Oz Series, Book 11)

The Lost Princess of Oz is the eleventh book in the Oz series written by L. Frank Baum. Published on June 5, 1917, it begins with the disappearance of Princess Ozma, the ruler of Oz and covers Dorothy and the Wizard's efforts to find her. The introduction to the novel states that its inspiration was a letter a young girl had written to Baum: 'I suppose if Ozma ever got hurt or losted, everybody would be sorry.' It was followed by The Tin Woodman of Oz (1918).
Dorothy has risen from bed for the day and is seeing to her friends in the Emerald City and notices that Ozma has not awakened yet. Dorothy goes into Ozma's chambers only to find she is not there.