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.
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.