Dafydd ab Hugh & Brad Linaweaver 
Endgame [EPUB ebook] 

They left behind everything that mattered to them– friends, lovers, country– to journey to the stars. Now Sergeant Flynn Taggart and Pfc. Arlene Sanders, USMC, have reached their destination… the homeworld of the demon invaders who destroyed Earth. But there, they find a scene of destruction that rivals any they left back on Earth. And suddenly, "Fly" and Arlene find themselves face-to-face with an even deadlier enemy than the demons they came to fight. The war for Earth is over. But the battle for the stars has just begun…