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 to keep the enhanced 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.