Forever Free: Forever War Book 3 by Joe Haldeman
William Mandela is a genetic throwback, one of the small group of humans who fought and survived the Forever War. They returned to find humanity has evolved into a group mind called Man. Seeking familiarity, the veterans seek to live a normal life on a harsh planet they've named Middle Finger. Man is happy with the arrangement, seeing the humans as a genetic failsafe, a baseline to return to if successive generations of Man corrupt its group mind. Surrounded by a society that is too autocratic and intrusive, living a dull existence which cannot compare to the certainties of combat and feeling increasingly alienated, the veterans plan an escape to the future by means of space travel and relativity. But when their ship starts to fail, their journey becomes a search for the Unknown, the elusive entity responsible.