The Emerald City of Oz

This is the sixth book of the Oz series. Because of financial difficulties, Dorothy's Uncle Henry has lost his farm in Kansas, and so Dorothy decides to accept Ozma's oft-repeated invitation to live in Oz permanently. Ozma transports Uncle Henry and Aunt Em to the Emerald City, and they too become residents of this wonderful fairy-land. Just about this time, Ozma and her people are threatened by the wicked Nome King who attempts to invade Oz and destroy the land. All ends well, but Ozma decides that, to protect her country from any possible uninvited guests in the future, Oz should from this time on be invisible to all those who live outside its domain. This of course means that the Royal Historian of Oz will no longer be able to communicate with the inhabitants, and so no further stories of Oz will be possible.