The Border by Robert McCammon
A SF romp that truly has echoes of McCammon's previous apocalyptic novel - Swan Song. This a straight forward action story with some mild ideological underpinnings. Two warring races - the Gorgons and the Cyphers - are in the process of destroying Earth which has the bad luck of being on the border of their respective races claimed territory - remain pretty alien. Which reduces their roles in the story to very traditional antagonists.
The real power of the story lies expectedly with the human survivors and their amazing ability to truck ahead under the most dire of circumstances. That, and a third alien party - who even though he is beyond human comprehension - reports to an even higher power, he in turn does not understand. Taking on the body of a young teenage boy near death - Ethan comes to learn and appreciate the worthiness of saving the human race.
So many terrible things happen to people and Earth that it becomes apparent early on that the story can only take one direction. And it does. Which both takes the tension down a notch but delivers the payoff a reader wants to see.
McCammon is always a fun writer to read. And I hear his next Matthew Corbett novel - FREEDOM OF THE MASK - clocks in at 180,000 words. Can't wait!