This question already has an answer here:
This was in a sample/preview chapter from another book that the publisher included at the end of something else, so I don't have much to go on, but here are the elements I recall -
- the characters lived in free fall in open space, with no need of environment suits.
- everything was in orbit around a very small sun, not very far away.
- unclear if breathable atmosphere extended to the sun, or was confined to a ring.
- the level of technology at hand seemed to be pre-industrial.
- the physical constants of the universe were different so that such a small sun could exist.
- material resources were running out, which seemed to imply the necessity of interstellar travel.
- I dimly recall something about coal or carbon being prominent.
The setup is similar in some respects to Larry Niven's Integral Trees, but it's definitely not that. In Integral Trees, the habitable spaces was a gas torus around some kind of ordinary collapsed star, but in this unknown work, the system center was a miniature sun which required a physically different universe altogether.
I'd like to read it, but I'm not sure I even still have the paperback that had that preview in it. Name that book, please!