The Forever War
The Forever War Series - Book #1
by Joe Haldeman
If you've read Old Man's War by John Scalzi, this will feel familiar. In fact, Scalzi himself realizes this in the foreword to the novel. He talks about how if he had read The Forever War, he wouldn't have written Old Man's War.
That said, while they are reminiscent of each other, they are two totally different adventures, and I thoroughly recommend you read both.
The Forever War starts with a fun premise: 70 year olds are encouraged to join the military. They sign up when they're 60, and they join up when they turn 70. Part of the lure? They're told they'll be young again.
You'll have to read it to find out what happens. It's a truly fantastic story, and it's clear why this is one of the sci-fi classics. Two thumbs up.