Bob Shaw had some cracking ideas.
In the Ragged Astronauts we enter into a binary planet system that is contained within the same atmospheric envelope. People live on one planet, simply called Land, and continue about their business giving little thought in general to Overland in the sky above. This changes however when their civilization is threatened by venomous floating spheres that spawn a contagion. A desperate plan is put in place to evacuate the population to Overland. Balloons are involved!
This is also a universe in which the ratio of a circle's circumference to its diameter is a nice round 3.
It's a thrill reading Shaw adventuring with big concepts, working through the details and their implications; and delivering action-packed set-pieces. Here the conceptual breakthrough, that ideal of SF, is satisfyingly present at the end.
Gollancz deserves applause for bringing these works back into (e)print. It would be such a shame to miss them.