from the back of the book:
John Perry did two things on his seventy-fifth birthday. First he visited his wife’s grave. Then he joined the army.
The good news is that humanity finally made it to the stars. The bad news is that, out there, planets fit to live on are scarce - and alien races willing to fight us for them are common. So, we fight. Far from Earth, the war has been going on for decades: brutal, bloody, unyielding.
Earth itself is a backwater. The bulk of our resources are in the hands of the Colonial Defense Forces, and everybody knows that when you reach retirement age, you can join up. The CDF doesn’t want young people; they want people who carry the knowledge and skills of decades of living. You’ll be taken off Earth, never to return. You’ll serve two years in combat. And if you survive, you’ll be given a homestead of your own, on one of our hard-won planets.
John Perry is taking that deal. He thinks he knows what to expect. But the actual fight, light-years from home, is far, far harder than he can imagine - and what he will become is far stranger.
SFSignal gives this a hearty recommendation: "Old Man's War, the first in a series I eagerly await, is a fast-paced, fun, tip-of-the-hat to Heinlein that succeeds in every way it can." SF Reviews starts by calling it "a tremendous, confident SF debut". SFSite's reviewer calls it "not bad, but simply rather pedestrian," saying he liked the first part of the book but that it later
seemed to drift into becoming a competently written, straightforward military SF adventure with a couple of interesting ideas thrown in about how a future society might persuade people to join the army.