The Ragged Astronauts: Land and Overland, Book 1
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.
Winner of the BSFA Award for best novel, 1986