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I don't have the picture but during one of the BTI listening parties, he was reading this and in his short, "Chicken and Futility," he was reading this
I would say that existential writers, especially Kierkegaard, J.D. Salinger (Catcher in the Rye - "Clapping for the Wrong Reasons"), and I'm guessing Albert Camus would be a good place to start.
Also, just look at all of the books he references in his songs. Kurt Vonnegut, for example, is mentioned in Crawl.