My worldview-changing book was probably Stranger in a Strange Land, when I was maybe 15. I also ready Brave New World around then.

I was raised in a super religious house; for whatever reason, that one or two books started my down a path of materialism/skepticism and away from religiosity/dualism/spiritualism. It was just the start, mind you. I was probably already primed and heading that way anyway, and that just happened to be the spark. I don't mean that there were any huge insights or great revelations for me there.