For me it’s definitely a book that often doesn’t get much love: Spin by Robert Charles Wilson. I read that book like ten times by now in both English and my mother tongue. Such a defining book for me since I first read it in my youth and it gave me a lot of food for thought regarding what it means to live a meaningful life. It is not really hardcore sci-fi after all but more a kind of coming-of-age novel that happens to take place in a sci-fi setting.