L. Frank Baum 
Glinda of Oz (Illustrated) 

Dorothy, Ozma and Glinda try to stop a war in the Gillikin Country. This was Baum’s last Oz book, and was published posthumously. Most critics agree this is Baum’s darkest Oz book, most likely due to his failing health. There have been many other Oz books released since, although not written by Baum.