Everything that Arthur Schopenhauer wrote.

It's a lot about finding purpose in life.

I started reading his stuff and then I became angry at him and at me.

At him because he showed me that I'm a slave of nature and at me because I started questioning life and humanity.

Then I started to disprove his thesis and found my own purpose in life, not only that, it also gave me a much better understanding of human nature, which can be used in marketing or product design.