The Brothers Karamazov by Feodor Dostoevsky. At the time I didn't realize that it changed me, but looking back on it, the perspectives I learned from it fundamentally how I viewed and understood other people.
It’s so good. I just finished reading the Constance Garnett translation and it’s just gripping all the way through. Well, I guess apart from Zossima’s life story in the middle, but that definitely has its place.
The Gutenberg HTML of it is definitely a labour of love and extremely high quality: https://www.gutenberg.org/files/28054/28054-h/28054-h.html . I converted it to a nicely formatted epub for Standard Ebooks and it’s in review now, so hopefully will be available this week.
Too many things. It implanted the idea that people, all people can be understood explicitly, through empathy (and that all people can be empathized with). It showed me a way of thinking that was based off of human emotions and understanding but also analytical. Father Zosima's conversation with the woman with the dead child also helped me resolve the question of how much of a part one's thoughts and intentions play in whether you're a good person, and in general what it means to be a good person and how that can change. There are other things, but on first reading those are the ways that it directly influenced my viewpoint (that I can remember now).