Glinda of Oz
Although this is the last book written by L. Frank Baum, it is one of the best. After visiting Glinda and finding out about the war between the Flatheads and Skeezers, Ozma and Dorothy decide to go intervene. On this truly magical adventure, a big chapter of Oz magic is uncovered.
You can buy this book through the Emerald City of Oz from the following publishers: