I've become really interested in the particulars of Existentialism lately. Unfortunately, I don't think my mind quite works in a way that is conducive to academic philosophy, so I'm not sure how much I could help you. As I understand it, a basic principle is the idea that the "self" is essentially a blank slate, and that any detail or fleshing out of the self is a construction of lived experiences (as opposed to any "essence" or "nature" of the self). I've just started to read a biography of Jean-Paul Sartre (Sartre: A Life, by Annie Cohen-Solal--it seems big and authoritative). Sartre is probably considered the father of Existentialism. It also seems to me that Existentialism is what skeptics might write off as "narcissism."
That probably wasn't terribly helpful, but this thread just jumped out at me, because it's something I'm interested in right now, too.