The Emerald City of Oz ( #6 In the Oz series) by Lyman Frank Baum

At the beginning of this story, it is made quite clear that Dorothy, the primary protagonist of many of the previous Oz books, is in the habit of freely speaking of her adventures to her only living relatives, her Aunt Em and Uncle Henry. Neither of them believes a word of her stories, but consider her a dreamer. She is undeterred, unlike her alter ego in the film Return to Oz, who is much perturbed by her guardians' doubts.

Later, it is revealed that the destruction of their farmhouse by the cyclone in the original book The Wonderful Wizard of Oz has left Uncle Henry in terrible debt. In order to pay it, he has taken out a mortgage on his farm. If he cannot repay his creditors, they will seize the farm. He is not afraid for himself, but both he and his wife, Aunt Em, fear very much for their niece's future. Dorothy arranges with Princess Ozma to take them to the Land of Oz, where they will be safe. Using the magic belt, a tool captured from the jealous Nome King Roquat, Ozma transports them to her throne room. They are given rooms to live in and luxuries to enjoy, including a vast and complex wardrobe. They meet with many of Dorothy's animal friends, including the Cowardly Lion and Billina the Yellow Hen.