Cities of the Air — Karl Schroeder

It is the distant future. The world known as Virga is a fullerene balloon three thousand kilometers in diameter, filled with air, water, and floating chunks of rock. The humans who live in this vast environment must build their own fusion suns and towns --enormous wood and rope wheels that are spun for centripetal gravity.
This complex and fascinating world is the setting for the novels of Virga, Karl Schroeder's interstellar far-future space habitat sealed off from contamination by the rest of the inhabited universe. These books have set a high standard for hard SF adventure in the last decade, and now the first two novels, Sun of Suns and Queen of Candesce, are brought out for the first time in one trade paper omnibus.