Emerald City of Oz - L. Frank Baum, John R. Neill
This sixth book was meant to be the last in the series. As a result Baum tied up a lot of loose ends which really helped strengthen the series later on when he returned to writing about Oz. In this book Uncle Henry and Aunt Em get to go to Oz. While Dorothy and her aunt and uncle are touring Oz the Nome King is plotting a full scale invasion of the Emerald City. Very entertaining read!