John Scalzi, Old Man's War, Tor, 2005.
Imagine a future where the human race must compete and fight with other species for planets to colonize. The Colonial Union takes colonists from third-world countries, and recruits soldiers from wealthy countries. But these are no ordinary soldiers. The recruits are all 75 years old, and they volunteer for ten years' service in exchange for the chance of a new lease of life as a colonist afterwards.
The story follows John Perry, who joins the Colonial Defense Force at 75. He is taken to a space station, where his consciousness is transferred into a new, improved, young body, cloned from his own DNA, but with some significant changes. He is now stronger, faster, and green-skinned. He goes through basic training and then fights in various battles, gradually learning more about the wider reality beyond earth.
The story is told from a military point of view, where there is no choice but to fight the enemy. This black-and-white attitude may upset some readers, but has to be taken as part of the world-building of this series. In one case, diplomacy is attempted with disastrous results. The life and experience Perry had on earth is hardly relevant to his life as a soldier.
Questions of identity are raised, but not answered: If you are given a new body, and your memories, experience and personality are no longer relevant to your new life, where you must change and adapt to new circumstances, and you can never return to your home planet, then in what way are you still the same person?
I have already read one of the later books in this series, Zoe's Tale, and now plan to read the next two. The only thing that bothers me in my reading of the series so far is the relative paucity of visual descriptions. The many alien species are given a few key features, but not enough to imagine them clearly in my mind.