This is a really hard question to answer. It depends so much on what your lived experiences and worldviews are – what book will expand, challenge, or flip your perspectives? Someone else may say some classic of the Western canon is the go to book for changing the way you think. And another may give you some recent social science tomes. Personally, I think the world of science fiction and fantasy can really help us think about our existence in new ways. I’ve been taken with Becky Chambers’ work recently. Anyway, maybe other passers-by will write notes with their own answers to this question?