Another vote for the Culture novels by Iain M. Banks. The man has a staggering imagination, a great sense of humour and he writes beautifully. The US versions used to have terrible covers straight out of the 1970s, though they seem to have been brought in line with the British editions now.
Alastair Reynolds, in particular his Revelation Space series, is very good if you're into 'harder' sci-fi. He used to work for the ESA and all his books contain at least semi-plausible interstellar space travel (no warp drives) and incorporates the travel time involved (and the time dilation effects of travelling at 90% of C) into his plots.
Joe Haldeman's Forever War is also good, though it does fall more into the military sci-fi bracket. The author based it upon his Vietnam experiences and the book describes a protracted, largely unexplained war against an unknowable race of aliens. The hero eventually comes home to find that he's completely alienated from his own society and that no-one knows or cares much about his experience.
Shameless plug time: My own sci-fi novel, Exiles, would fall into this genre as well. Currently available on Amazon US, UK and Smashwords.
https://www.amazon.com/dp/B003UYUWE2
https://www.amazon.co.uk/dp/B003UYUWE2
https://www.smashwords.com/books/view/18959