Yes, there are.
Some editions of the book have a chunk edited out of the middle where William and Marygay return to Earth together. William stops a rape-in-progress when the pair goes sightseeing in London, and they meet Marygay’s parents on a farming collective somewhere in the American midwest.
If you have Haldeman’s collection of short stories entitled Dealing in Futures, the missing section is reprinted there as “You Can Never Go Back”.
Additionally, Haldeman also penned a short story called “A Separate War” that deals with what Marygay went through when when the military reassigned both her and William following their return to Earth after their (disastrous) second tour.
Forever Free is the direct linear sequel to The Forever War. William, Marygay, and the other veterans living on Middle Finger are discontent with living under the direction of “Man”, and decide to use one of the mothballed battlecruisers left over from the war to travel 40,000 years into the future via time dilation. They hope that when they come back, things will be different … perhaps better.
Forever Peace, even though it sounds like it should be the sequel to The Forever War, is not.
Haldeman is a Vietnam vet (combat engineer, wounded in combat). It was clearly modeled on Heinlein’s Starship Troopers (although Haldeman is a huge fan of Heinlein and has denied that it is an “answer” to Starship Troopers.)
When it first came out, the idea of a co-ed military unit was considered bizarre, as was the open homosexuality of the population in later years. It was also one of the first, if not THE first, novels to propose using black holes (called “collapsars” in the novel) as portals for crossing vast distances. Other novels (like Poul Anderson’s “Tau Zero”) have portrayed the effects of FTL travel, but the disorientation of changes in culture and technology was well-handled by Haldeman.
I think it’s one of the best SF novels of the 1970s.
Haldeman wrote an interesting non-SF, somewhat autobiographic novel about his Vietnam experiences, “War Year.” It has an unusual style, which was due to having been written on contract for an adult literacy program to teach illiterate adults with limited vocabulary, but with adult subject matter.
The Forever War is terrific, but I didn’t care for the sequel at all. The short story “A Separate War,” which psiekier mentioned, is very good, and a worthwhile tangent/supplement to the novel.
If you like Haldeman’s military sf, you’ll also want to check out John Scalzi’s Old Man’s War series. Great reads.