Land and Overland Series Series by Bob Shaw

Land and Overland - twin worlds a few thousand miles apart. On Land, humanity faces a threat to its very survival - an airborne species, the ptertha, has declared war on humankind, and is actively hunting for victims. The only hope lies in migration. Through space to Overland. By balloon. The Ragged Astronauts - first volume in an epic adventure filled with memorable characters, intense action, engaging notions, exotic locales.

Twenty-three years ago, the inhabitants of Overland left Land to escape the Plague. In this sequel to The Ragged Astronaut, the survivors of Land bring an ultimatum-- submit or die of the Plague.

The grandson of the hero of "The Ragged Astronauts and The Wooden Spaceships" must find the reason why a crystalline disc is growing between the worlds of Land and Overland, and then try and stop it.