The Emerald City of Oz by L. Frank Baum
The Emerald City of Oz is the sixth of Baum's 14 Land of Oz books. Originally published on July 20, 1910, it is the story of Dorothy Gale and her Uncle Henry and Aunt Em coming to live in Oz permanently. While Dorothy and family are toured through the Quadling Country, the Nome King is assembling allies for an invasion of Oz. This is the first time in the Oz series that Baum made use of double plots for one of the books. This book was also adapted into a Canadian animated film in 1987, but I haven't been able to find this to watch it.
Baum had intended to cease writing Oz stories with this book, but financial pressures prompted him to write and publish The Patchwork Girl of Oz - next week's book - plus 7 more Oz books. An interesting fact about Emerald City is revealed in this book: Emerald City is described as having exactly 9654 buildings and 57,318 citizens. We didn't see all this in the 1939 movie The Wizard of Oz, did we??!!