The Forever War by Joe Haldeman Books 1-2 in the Series
The 2-book set of mixed paperback novels includes the below titles:
The Forever War (ISBN 9780312536633): The Earth’s leaders have drawn a line in the interstellar sand—despite the fact that the fierce alien enemy that they would oppose is inscrutable, unconquerable, and very far away. A reluctant conscript drafted into an elite Military unit, Private William Mandella has been propelled through space and time to fight in the distant thousand-year conflict; to perform his duties and do whatever it takes to survive the ordeal and return home. But “home” may be even more terrifying than battle, because, thanks to the time dilation caused by space travel, Mandella is aging months while the Earth he left behind is aging centuries.
Forever Peace (ISBN 9780441005666): 2043 A.D.: The Ngumi War rages. A burned-out soldier and his scientist lover discover a secret that could put the universe back to square one. And it is not terrifying. It is tempting…