Irrational Man: A Study in Existential Philosophy

Anything by Douglas Rushkoff. I particularly enjoyed Survival of the Richest: Escape Fantasies of the Tech Billionaires. Another good one is Irrational Man by William Barrett. The only truly comprehensive look at existentialism I've read.

Irrational Man is a good introduction to existentialism and he has a very readable overview of philosophy from the Greeks until now. This is will largely be ignored on this tread, but The Bible is a source of philosophical thought for millions of people. You can get a clear picture of the central theme by reading the book of John.