Some years later, Haldeman wrote two other novels linked to The Forever War, though only one is a direct sequel.
The sequel is Forever Free, in which Mandella, with his wife and children, is now a colonist on the icy world of Middle Finger. When they try to use time dilation effects to escape the post-human hive mind known as Man, things go wrong, and they end up returning to a depopulated planet, meet an alien shapeshifter that has coexisted on Earth throughout history, and end up in a face to face meeting with God. It is nowhere near as good as the original, but it is interesting as a sequel.
Much better, but only tangentially connected to the original, is Forever Peace, which also won the Hugo, Nebula and John W. Campbell Memorial Awards. This is another novel which argues that war is an aberration, but in this case it is a war here on Earth fought by armies of robotic "soldier boys" who are controlled by plugged in operators. However, it is discovered that being plugged in like this cures all warlike impulses, so that the very act of fighting the war ends war.
If you love the military action (and suit to suit combat) of Forever War, read the classic Starship Troopers by Heinlein. While Forever War is an argument against war (and specifically, the Vietnam War), Starship Troopers is the celebration of all things war. Both have a shit load of action. And if you want a novel that straddles the middle between Starship Troopers and Forever War, then give John Steakley's Armor a good read.
For a somewhat different take on future wars, you should also check out Old Man's War by John Scalzi in which it is old people who have already lived productive lives who are recruited to fight and are then given enhanced bodies. But this is still an anti-war novel, the characters are psychologically damaged by their experiences and it is far from clear that the humans are fighting on the right side.