Mad Empress of Callisto

Earth-born Jandar and Princess Darloona are kidnapped by the ambitious Empress Zamorra of Tharkol, who is under the influence of a sinister Kuurian Mind Wizard, forcing them to escape through the dangerous landscapes of the Jovian moon.

Rescued from a dungeon by the thief Glypto, Jandar kidnaps Empress Zamorra, but their balloon crashes, leading to their capture by the insect-like Yathoon horde. The group navigates dangerous encounters, including beast-riders and military caravans, before realizing the Empress may not be as mad as believed.

Book 4 of the 'Callisto' series continues the tale much as you'd expect. Carter's usual contrived plot devices are starting to take their toll though.