Raft by Stephen Baxter
Rees is a miner, living in a nebula surrounding a black hole, in a universe where gravity is a billion times stronger than in our own. The humans in this universe are the descendants of the crew of a spaceship which fell through a wormhole from our own universe many years before.
As a consequence of the high gravity, the nebula is running out of oxygen and dying. Rees and his compatriots must figure out a way to leave the nebula and start a new life in a neighbouring, oxygen-rich nebula before their old home dies.
Though he is well known for his hard SF stories, I found this Stephen Baxter novel a tad too hard. The entire novel just seemed like a travelogue, with each chapter introducing a different part of the nebula or a different nebula life form and throwing out large chunks of exposition to explain the science of the high-gravity universe. The characters were not fleshed out enough, and the plot was very thin. As the first book in the Xeelee sequence, here's hoping the next book in the series improves, now that the universe has been set up in this first novel.