This is the second book in a series begun with the book Eon.
First, let me make disclaimers. To appreciate this book, it probably helps to have read Eon. Although I have read Eon, it was some time ago and my recollection of it was not enough to help me very much. Also, it seems to me there are some stories that resonate with an individual more than others. At least for well over 100 pages, most of the story didn't resonate much with me. As a result, I may be at a disadvantage to appreciate Eternity.
For an extended period starting at the beginning of the book, there are a number of sub-plots that seemed to me to plod along in isolated story lines. There were glimpses of potentially big SF ideas, but not enough to compensate for a story that seemed to take a long time to draw together. Meanwhile, when potentially interesting SF speculation peeked out in places, there was uncertainty whether things were what they appeared - or if it was something other than what it might appear (and therefore might be less interesting).
I read the book because it was the Yahoo / Goodreads Hard SF group's Feb. 2012 selection. If it had not been for that, I may have put this book down and tried something more to my taste.
Later in the book, the plot threads do merge. At that point, the book gain a more cohesive feel - a less unfocused impression. The part played by the big SF speculations increases some. Yet, we never fully visit those most interesting ideas. On the whole, the big SF possibilities are like secondary characters, rather than central characters who spend most of the time on stage. What we do see of these ideas aren't explored deeply enough to say how plausible their core concepts are. There were certainly some details related to them which struck me as doubtful.
The book Eon tells of humans opening an entrance to "the Way" - something like a long tube outside of normal space-time which included connections to other places. An alien race comes traveling down the Way seizing control as it progresses. Retreating humans end up closing off the connection between the Way and our Solar System. In Eternity, a heavily damaged Earth has humans living at a low level of technology. High tech humans live in space. The high tech humans try to improve conditions on Earth, but those on Earth tend to resent them. Sub-plots include a man living in a space colony finding a secret facility where the mind of one of the invading aliens was stored long ago. On another planet with an alternative history to Earth's, a grandmother gives a young woman high tech devices that can connect to the Way. An entity that is derived from a Russian who participated in the war with the aliens in the Way comes to bring a message to humans.