The Lost Princess of Oz by L. Frank Baum

Who is stealing all the magic in Oz? Dorothy and her friends set out to comb all of Oz, not only for magic stolen from Glinda and the Wizard, but also for the kidnapped princess, Ozma. Along the way, they explore regions never seen in other Oz books, meeting strange and interesting people and animals, and falling into peril more than once. It’s a desperate mission – for if the thefts are all linked, then it means that some magician unknown to them has acquired powers beyond any available to them now. How will they find him? And how will they conquer him? Not one of them knows – but with continuing faith that goodness will triumph, they march forth to try.