From the book:

Well, my dears, here is what you have asked for: another "Oz Book" about Dorothy's strange adventures. Toto is in this story, because you wanted him to be there, and many other characters which you will recognize are in the story, too. Indeed, the wishes of my little correspondents have been considered as carefully as possible, and if the story is not exactly as you would have written it yourselves, you must remember that a story has to be a story before it can be written down, and the writer cannot change it much without spoiling it.