Old Man's War

John Scalzi channels Robert Heinlein (including a wry sense of humor) in a novel about a future Earth engaged in an interstellar war against more advanced species. Citizens volunteer for the Colonial Defense Forces after retirement, in exchange for which they have their consciousness transferred into a young body, cloned from their DNA but enhanced. If, against the odds, they survive two years of combat (or 10 years if things aren't going well, which they're not), they get another body and enjoy a fresh start on a colony. This is Scalzi's first novel, and it creates a future he will revisit in subsequent stories.

John Perry did two things on his 75th birthday. First he visited his wife’s grave. Then he joined the army. The good news is that humanity finally made it into interstellar space. The bad news is that planets fit to live on are scarce—and alien races willing to fight us for them are common. So: we fight. To defend Earth, and to stake our own claim to planetary real estate. Far from Earth, the war has been going on for decades: brutal, bloody, unyielding. Earth itself is a backwater. The bulk of humanity’s resources are in the hands of the Colonial Defense Force.

Synopsis from Open Library.