A Cidade Maravilha de Oz, por John R. Neill

De Wikipedia :

Yet her ambition lures her into running against Ozma in an Ozlection to become ruler of the Land of Oz. It is clear that Ozma will win any fair election by a landslide — but a landslide is a terribly dangerous thing to have in the Emerald City. So Prof. Wogglebug cooks up a more random choice, in which citizens are weighed on scales to determine their votes.

Descobri a lista de livros de Oz na Wikipedia, e depois encontrei um que parecia coincidir com a descrição de "Ozma não se tornando mais a régua", e fui para essa página ... e voila.