The question in the thread title was posed by Lifehacker (here
) earlier today, and I thought it might be interesting to read what books the people here considered to have shaped their lives.
For me, I think it was probably The Hitchhiker's Guide to the Galaxy. I read it (and the sequels) in junior high and the style of humour really stuck with me. What stuck with me more than that though was the suggestion in the narrative that everything that was happening was entirely random and entirely pointless. I think it was probably that seed that got me interested in critical thinking, reasoning and the philosophy of free will.
Other than that, I think the Arthur C. Clarke books I read around the same time probably had, collectively, a similar impact.