And I finally got my hands on the last Random House "Brand New Oz Adventure" book! Well, actually, the copy I wound up with was a British edition published by Puffin. Still, same story, same illustrations.
Dorothy and the Magic Belt was by Susan Saunders. I can't find much but bibliography for her. She mainly seems to have contributed to small book series.
Anyway, her contribution to this little series: Dorothy goes to Oz abruptly while explaining how she signals Ozma to Aunt Em and Uncle Henry. While Dorothy takes the Sawhorse on an excursion in the Munchkin Country, a young boy plans to get back on Ozma: Dr. Nikidik's son, referred to as Nikidik the Younger or just Nik. He wanted to be a sorcerer, like his father, but since unauthorized magic in Oz is now illegal, Nikidik to Elder has agreed to follow the law.
Still, Nik wants to do magic, so what can he do? Go to the Emerald City and take the Magic Belt! He accomplishes this with "Youthing Powder," which makes people younger upon contact. When he makes all the people in the Emerald City into younger versions of themselves, taking the Magic Belt is a piece of cake! However, Tik-Tok manages to see what's going on.
Can Dorothy, the Scarecrow, the Tin Woodman and Jack Pumpkinhead get the Belt back from Nik and his aide, a youthful Mombi?
The story fails to fit in continuity about Mombi, Aunt Em and Uncle Henry, and Dr. Nikidik in the later Oz books. A new future for Mombi is written that doesn't fit in with what Thompson wrote and what the Scarecrow suspects in The Tin Woodman of Oz. Aunt Em and Uncle Henry are contacted by magic, meaning their skepticism about Oz in The Emerald City of Oz should be completely unfounded. As for Nikidik, this story's conclusion falls flat given we're told that Nikidik died later, and if you follow fan speculation that he was really Dr. Pipt, where's his son when we meet him in The Patchwork Girl of Oz?
David Rose's illustrations are still as good as ever, but now that I've seen four books, to be honest, I've seen Oz artwork I prefer over him. Not that he's bad, but he's got nothing on Denslow, Neill, or even Dick Martin.
Overall, I suppose the completist book collector would want this, but as it stands, it's just not a very great story.