Bob Shaw 
Ragged Astronauts [EPUB ebook] 
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