Review
Old Man's War, John Scalzi, 2005, ISBN 0765309408
Mankind has started to spread out in the galaxy, and so have a lot of other races. The available real estate is scarce, which leads to near-constant war for land.
The only way for Americans to get into space is to join the Colonial Defense Force (CDF). They guard human colonies, and go to war over disputed planets. The CDF only takes people who have reached their 75th birthday. A vague promise of being made young again is a pretty strong incentive to sign up. The catch is that joining the CDF is a one-way trip. If you survive your tour of duty, hardly a sure thing, you will spend the rest of your life on some colony planet; returning to Earth is not an option.
John Perry signs up. He just turned 75, his wife, Kathy, died several years previously, and his one adult son lives on his own. On the spaceship taking him, and several hundred others, to basic training on another planet, he learns just what the becoming young part is all about. His consciousness is transferred into a cloned body, in its mid-twenties, made from his own DNA, which was extracted from him several years previously. It's very much of a new and improved body with a green skin color. He also has a computer implanted in his brain, which can talk to him and communicate with anyone else.
After basic training, Perry and his squad travel from planet to planet. Friends die, and new friends are made. During one disastrous operation, Perry crash lands on a planet, and is rescued by. . . his wife. She too is green, but the resemblance is way too close to be a coincidence. She (her name is Jane) is part of the Ghost Brigades, actually clones of dead people. Having no conception of what life is like as a realborn, they are kept far away from the rest of the CDF. Perry is made part of a Ghost Brigades squad, and begins to tell his squadmates what it's like to be married, and to love another person.
Here is an excellent novel. It has space travel, it has weirdness, it has heart and it has a lot of great writing. This is highly recommended.