My greatest influence so far has been Michel Foucault. After reading his The Hermeneutics of the Subject, Society Must Be Defended, History of Sexuality I to III and his other works, my views on myself, the society I live in and other people have become much more aware of the complexity that mutual influences, history, society, education, sexuality and so on have on each other and most importantly on a single individual.
All of the big ideas I'm interested in like identity, politics, history, nations, subject, education stem from reading Foucault. I find his writings both compelling and interesting to read.
I understand I'm not very objective here, being a philosophy major, and doing my MA on Foucault
Oh yeah I forgot. Fiction wise...
When I was in high school one book shook me to my core, because I completely identified with absurdity of society K was living in and my own society. I couldn't stop thinking about this book, and I re-read it multiple times.
The Trial by Franz Kafka. It's translated as Process in my language.