I like this series a lot. I adore Pathfinder, and Ruins is a good sequel, but I'm not really sure what happened here. 
 There's a lot of talking. Oh, yes, lots of talking. And a lot of space travel science, which is cool, even if most of it goes over my head. There's a whole lot of talking and not a whole lot of "doing," which is okay in the beginning as a series refresher but tedious for the whole 600 pages. 
 There's a lot of filler here, including most of what happens on the Earth sidequest. And the interesting bits fizzled out until I completely forgot they were a thing--like, Rigg was going to explore the other wallfolds, but we didn't actually see all of them. This could have been interesting. And there are preparations for war (on Garden and beyond), but there's no fighting? Did I really just read about preparing for war, and war councils, and discussions about what they're going to do and not actually see the result? 
 I'm giving this three stars regardless, because I like the characters (even if they're whiny children, even after they grow up). And I'm biased toward time travel, and there is a lot of talk (again, with the talking) about the risks of altering the past. Just don't stop to think about all the alternate timelines, and whether or not they exist after the past is altered. Because that stuff messes with your brain. 
 There are a lot of loose threads at the end, despite the seemingly neat conclusion. I'm a fan of open-ended conclusions, but there's a difference between that and this feeling of incompleteness. I thought maybe there would be a fourth book to truly finish it, but nope. I'm not sure I would've read it, anyway.