The tenth book in L. Frank Baum's Oz series is the very non-Oz Rinkitink in Oz (1916). We can blame the lack of Oz in this Oz book both on Baum's documented general lack of interest (except of course, financial interest) in the long-running Oz series, and the fact that the bulk of the book is actually an adventure story that Baum wrote in 1905.
To make up for the lack of Oz in the story itself, Baum begins the book with a geographical placing of the islands of Pingaree, Regos, and Coregos (where the story takes place) in relation to the land of Oz. They are kind of nearby it. He also ties the story in with the land of the Nome King (who we have visited before), and as always ends it up with a big banquet in the Emerald City.
Although I think the lack of Ozzyness is a little silly in an Oz book, Rinkitink of Oz is actually a very nice little adventure story. It involves a young prince of a prosperous island nation named Inga. His parents and all the citizens and wealth of the country are taken away by evil warriors while he is taking a nap up in a tree. He then sets off with the pudgy king of a neighboring island (that's King Rinkitink!) and his talking goat, along with three magic pearls he got from his father, to save his parents and his people from working as slaves in the mines and fields of King Gos and Queen Cor.
This quest eventually leads him to the land of the Nome King where he has to save himself in various trials as he works to free his parents. Just when things are getting exciting, Dorothy gets beamed in by Ozma to save the day and bring everyone back to Oz for the obligatory banquet. This time we get a laundry list of our favorite characters, but no real interaction between the new characters and the old group.
This story follows Baum's familiar pattern of a young child and an adult to which he or she is not related going out in the world to save something or somebody and meeting up with danger and adventure along the way. And obviously this formula works...
[Read the whole thing here. Do it! Or else just read the Wikipedia description, you lazy thing.]