Taking place outside of the boundaries of fairyland, death can actually occur, so there is already an edge of danger to the adventure. The magic pearls provide all of the power and protection and wisdom the Prince needs, but their nature allows Baum to dispense with them where needed which keeps the action interesting. The only issue is that inevitably Oz must come into the picture, but unlike in The Scarecrow of Oz where Glinda dispatches the Scarecrow with some actual help and authority, backhanded aid comes from Oz simply because Dorothy felt like it at the time. Our erstwhile heroes, in the end, were deprived of their chance to triumph in their own adventure. Inga could have done it if he'd just been given a chance.
Frustrating as the end was, I can't go on being so disagreeable about these books considering I have four or five more to go before I can walk away a free man. Baum had a story that carried itself all the way through with bits of Edwardian slapstick and rhyme humor, adversity overcome and surprising turns. This was a book that Baum was happy to be writing. Until he had to slap the word Oz on the cover and dispatch a farm girl and a basket of eggs to sweep away any progress the story had made towards a satisfying conclusion. So I'm pretending that never happened and rating the book as well-liked. I admit there is irony in complaining about the intrusion of Oz when I'll likely never read any of Baum's non-Oz books, because from the energy that clearly remained in him they wouldn't be a waste of time.