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I read this book many years ago. It is set on a double planet with a shared atmosphere. The people on the planet have a caste system, they are either military or thinkers. There are giant trees on the planet, which are used for almost everything, building materials as well as an energy source from the crystals the trees form.

There are also poisonous bubbles that float around. The inhabitants were able to destroy them with ranged weapons, but the bubbles became more dangerous which forced the mass migration of the inhabitants to the adjoining planet via balloons made from and powered by the giant trees.

There is also a sequel where the main character from the first book travels through space to another planet in the system using a more advanced rocket based off the trees.

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The Ragged Astronauts (1986) - part one of the Land and Overland trilogy by Bob Shaw.

From Google Books:

The first in a three-book series, The Ragged Astronauts introduces the twin worlds of Land and Overland, which orbit only a few thousand miles apart and share a common atmosphere.

From Wikipedia:

Land is a strictly feudal society that undergoes a peak energy crisis (the trees that provide energy and hard materials are scarce), and is undergoing a process of cultural decay. Much of the human population of Land travels to Overland via hot-air balloon to escape airborne creatures called the Ptertha.

  • 1
    @TheLethalCarrot thanks for the formatting and tidy up... Can't do that on mobile – Happy Robot Jan 9 at 15:11

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