From the book:

My friends the children are responsible for this new "Oz Book," as they were for the last one, which was called The Land of Oz. Their sweet little letters plead to know "more about Dorothy"; and they ask: "What became of the Cowardly Lion?" and "What did Ozma do afterward?" - meaning, of course, after she became the Ruler of Oz. And some of them suggest plots to me, saying: "Please have Dorothy go to the Land of Oz again"; or, "Why don't you make Ozma and Dorothy meet, and have a good time together?" Indeed, could I do all that my little friends ask, I would be obliged to write dozens of books to satisfy their demands. And I wish I could, for I enjoy writing these stories just as much as the children say they enjoy reading them.