Book — The Well of Stars by Robert Reed

When I happened upon Marrow, the first book I'd seen by this author, I was quite amazed — this was a confident look into a distant future (10^5 - 10^6 years hence) purely relativistic universe, with emortal post-humans perfectly adapted to that state : when a few are marooned on a barely habitable planet, their escape plan involved bootstrapping a technological civilization over the next five kiloyears.

The next books of his I found (Sister Alice, Down the Bright Way) were earlier, and less impressive, adequate but nothing more.

And then this — a return to the story that began in Marrow, with the Great Ship, itself the size of Neptune, facing an enemy that can overwhelm even its vastness, and possibly the fate of the entire Universe in the balance. This book builds upon its predecessor and is simply breathtaking.