I think I'm gonna start here. Thanks
I would not start with The Forever War. Its an influential classic, but that doesnt mean that its something that today's reader is likely to enjoy as much as some of the popular current stuff. Here is a 3 star review I did of The Forever War on Amazon:
The basic premise of this book is known to anyone that has read the back cover -- soldier who, because of relativistic effects, fights "forever". As one would guess, the soldier comes back to human society several times, with big gaps in years.
Because the book was written a while back, the first time the soldier comes back to Earth is more or less now, and the described world (and thus the marine's future shock) doesnt feel too realistic. To be fair, you have to evaluate it for potential realism as a parallel timeline given that the author was writing 30ish years ago, but even at this level, the sheer magnitude of change in such a short time didnt feel right to me. Maybe its possible the entire world could sink back to wild-west type chaos in less than a generation after a "collapse," but I didnt really buy it here. I suppose the point is more the future shock than the future itself, and on that level it works.
The second time he comes back, we really arent even told too much about what the world is like, just about sexual preferences, which is dissappointing (and a little comical -- I mean seriously, 400 years and all the author can talk about is sex, and people otherwise act and look the same???). In fairness, perhaps dramatically changing sexual practices was just another way to ram home the future shock concept, which, again, seems more central to the book than the actual futures.
The third time he comes back, your given only a brief glimpse of what the human universe is like at this point, and not told how we got there. Thus, the future elements are very light, and much of the book, is simply what its like to be a marine in a long bloody war, through training and fighting. The combat sequences are not going to overwhelm anyone, but they do keep the book moving. Characterization is also very light and there is no real character "development" to speak of -- the protagonist is likable enough, but not terribly distinctive, and he doesnt change or grow during the book.
In sum, the Forever War is not hard sci-fi with detailed predictions about the shape of future technology and events. Its soft, concept driven, sci-fi that ran with the concepts of a pointless war stretching forever and future shock from relativistic effects, and included some military stuff and sex to keep the story moving. You can see where in the era following the Vietnam war it was considered a meaningful book, but one could wish that there was a bit more too it. Similarly, if this was one of the first books to meaningfully explore future shock, you can see where that would be influential as well. Reading today, however, future shock is old hat to me, and I'd really like to see some believable or interesting futures to go along with the concepts.
The book is mostly a quick breezy kind of read, with just a few slowish segments where Haldeman makes some token effort to describe the physics of a space battle, or the history of a future earth time. So it moves pretty well, and even if it doesnt become a favorite of yours, reading an influential classic (which this is) for a good price cant hurt ya any.