Mystery in the Skies — Charles A. Johnson

The Super Sleuths Seven are en route to Europe, thanks to a contest that Heatheria Winchester entered over a year ago--and they're flying on the most luxurious airship this side of Mount Melania, the Hispaniola. However, many hours later, during a magic act run by one of Roxy Norrington's acquaintances, the priceless Gallstone Ruby of Burma has mysteriously disappeared--without the magic act being completed. To make matters worse, the magician who was performing the act--Lioness--has mysteriously disappeared along with it. All of these events seem to point to an angry guardian of the missing ruby, and the Super Sleuths must band together to capture him, getting help from Lioness's two performance assistants. Will they succeed, or will the Gallstone Grundel exact retribution for his missing ruby?