This is a hard. Books that made me a better thinker have to be the hard ones / ones that made me change. Top two would be
Prolegomena to Any Future Metaphysics, Immanuel Kant.
Supposedly more accessible version of Critique of Pure Reason but still very hard and mind-bending for me at least. Not just philosophy was easier after wrestling with this content.
The Structure of Scientific Revolutions, Tomas Kuhn.
Made me self-aware about what scientific thinking actually is.
Prolegomena to Any Future Metaphysics, Immanuel Kant. Supposedly more accessible version of Critique of Pure Reason but still very hard and mind-bending for me at least. Not just philosophy was easier after wrestling with this content.
The Structure of Scientific Revolutions, Tomas Kuhn. Made me self-aware about what scientific thinking actually is.