Joe Haldeman 
Starbound [EPUB ebook] 

After the shocking first contact between humans and alien life on Mars, Carmen Dula and her husband board a tiny, long-range craft with five other humans and two Martians. They travel to a distant solar system that is home to the "Others" — an enigmatic, powerful, and possibly immortal race. Once there, they manage to find enough common purpose to forge a delicate truce between human, Martian, and Other.By the time Carmen and her party are sent back to Earth, fifty years have passed — and the Earthlings have not been idle. They have built a massive flotilla of warships to defend Earth against the Others’ expected aggression. But the Others have more power than any could imagine.And they will brook no insolence from the upstart human race…