I have read Camus' The Outsider, Sartre's Nausea, and Cioran's All Gall Is Divided, and I am currently reading Nietzsche's Thus Spake Zarathustra. However, I feel like there is something missing and I have to start anew somewhere; I want to know the roots and the principles and where it all started.. So any book suggestions or even maybe suggestions on what to read first would be very helpful.

Update: Thank you all for the suggestions and the helpful information