Books that actually changed how you think?

I’m looking for a book that stays with you long after you finish the last page. I feel like I’ve been reading a lot of "fast food" books lately, entertaining in the moment, but I forget the plot two days later. I want something that genuinely shifts your perspective or makes you see the world a little differently.

What is the one book that actually changed the way you think? I’m open to anything from fiction to philosophy, as long as it’s a total "brain reset."