[erur]

This is very basic but I read both at a young age and I'm massively happy I did.

1st one being "Thinking fast and slow" by Daniel Kahneman - completely reshaped my model of how much we are in control of our thought process.

2nd one was "Thus spoke Zarathustra" by Nietzsche - which is a very difficult read. Overall Nietzsche's approach to philosophy just immediately resonated with me and sent me down a path of thought that's still the foundation of my world model today. The whole approach of seeing life as inherently meaningless so meaning can be chosen at will was a great thing to have growing up. You definitely don't need to read the book to get there though - just reading a summary of what Nietzsche is about will probably be enough.