The Forever War is a classic of hard SF that I enjoyed back in the day. I read We a couple years ago and really enjoyed it (the writing is great even in translation). Odd John has to be a -rec; I like Stapledon but Odd John is redundant with his other stuff and not very

Haven’t seen anyone mention The Forever War by Joe Haldeman. From what I remember this is core to the premise - ships travelling at relativistic speeds encounter enemy ships that might be decades or more ahead or behind in technology depending on when they launched.