The first book of the series, "Marsbound", was excellent...but it should have been left alone. The second and third felt like movie sequels - needed to continue the story for popularity/profit reasons, but the author really didn't have much left to say, or much interest in saying it. The second, "Starbound", was okay, but felt like a transitional story. The third, this one, was very disappointing. After going on in an uninspired "Post-Apocalypse" form, it just....stops. One gets the distinct impression that the author was just sick of it all, wanted to get on with other things, and just created a Deus Ex Machina of the worst sort just to have some sort of quickie resolution. I suppose he could claim that was the point of the arc of the story, facing the caprious and godlike "Others", and that it was intended to be more a character study and evocative of life, which often is capricious and just ends....but if that were the case, the character development is just too shallow to qualify. If it's to be "space opera," then do that justice!
The author is here unfair to to his readers, as we invest time and commitment (not to mention, money) in the main characters, only to have them cursorily tossed aside. Perhaps there will be a fourth to have a better resolution - and frankly, to make it up to his readers - but I'll be very leery of it, and his other works, after this...which is a shame, because when he's really engaged, as in "Marsbound" and "The Forever War", he's a master.
If you must read these, borrow "Starbound" and "Earthbound" from your local public library, and save your money.