Bob Shaw SF Gateway Omnibus: Orbitsville, A Wreath of Stars, The Ragged Astronauts

Orbitsville: Racing from the certain vengeance of Earth's tyrant ruler, space captain Vance Garamond flees the Solar System, and discovers the almost unimaginably vast spherical structure soon to become famous as "Orbitsville"—a new home for Earth's huddled masses. 

A Wreath of Stars: Thornton's Planet is an anti-neutrino planet detected on its approach to Earth. It can be seen only through the newly developed magniluct lenses and its arrival causes a wave of panic. When its course carries it past the earth, interest in Thornton's Planet wanes—but the visit of Thornton's Planet has had effects on Earth further-ranging than anyone could have imagined. 

The Ragged Astronauts: Land and Overland are twin worlds a few thousand miles apart. On Land, humanity faces a threat to its very survival, as 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.