Raft by Stephen Baxter
This book is listed as being the first book in the Xeelee series.
I have mixed feelings about Raft. Some of them may be about Raft itself, some of it may be based on my expectations. For instance, I've heard about Baxter's Xeelee books but I never before read any of the books. Raft is considered the first book in the Xeelee series. So, I expected Raft to include the Xeelee. However, except for references to the human characters’ ancestors having dealt with a Xeelee artifact, the Xeelee are absent.
Also, my dislike of scientific inconsistency made the first 1/4 of the book awkward. There were various references that seemed inconsistent with the laws of nature. Finally, 1/4 of the way into the book, an explanation was provided, but by that time I already had reservations about the book.
However, part of it was also my reading preferences. Those of you who have followed my reviews may recall my reaction to Robert Reed's Marrow. To me, the artifact in Marrow was such a marvel, but most of the book was about squabbling among the humans. That might be realistic, but once you spark my interest in big tech or science it seems a shame to make it merely a backdrop for human foolishness. Similarly, Raft provides a view of a different cosmos and diverse groups of humans. On the other hand, a fair amount of the book is the human squabbling among various groups in various ways at various times. Even when the groups of humans aren't in conflict, we're seeing the too-common theme of humans living among the remains of their ancestors’ technology, while the current generations have only partly retained the knowledge and tech.
The humans are living in a breathable nebula cloud, with no planets or such to inhabit. In that sense, it's somewhat reminiscent of Niven's Integral Trees. And while the humans live on man-made constructs, not giant "trees", there are large "trees" living in the nebula. (And the humans have learned to use the "trees" for transportation within the nebula.)
The story begins in a mining community encircling a 50-yard wide ball of iron. The relatively flimsy ring-shaped mining habitat reminded me of the towns in Schroeder's Sun Of Suns. (Of course, Raft was actually written before Sun Of Suns.) Conditions are getting worse in the mining community. Rees wants to find out why, so he sneaks a ride to "the Raft" - sort of "the city" in the nebula. The Raft is a larger, sturdier construct than the mining town, and people face fewer hardships there. However, conditions are declining there as well.
The nebula is becoming less hospitable to life. Scientists on the Raft are trying to understand this and find a solution. However, their progress is slower than the rise of discontent among those whose lives are becoming harder. The less educated vent their anger and remove the previous leaders from power. Many scientists, including Rees, are shipped to the mining town as punishment. They face resentment and mistreatment there. Eventually, Rees is exiled to live with a reviled group of humans (“bonies”) living elsewhere. We see the disagreeable conditions among those people. After a while, a flying "whale" comes by and Rees manages to ride it away.
While riding the whale, Rees sees a possible way to save the humans in the dying nebula. Rees returns to the Raft to tell of his discovery. When he arrives the Raft is at war with the miners. There are various issues and conflicts while trying to implement Rees' solution. It requires the three different human groups (miners, Raft and bonies) to work together, despite their past conflicts.
Although there are some fascinating parts to this universe, the book may appeal more to readers of the humans-with-devolved-tech style of SF.