The Demon-Haunted World: Science as a Candle in the Dark, by Carl Sagan.
It changed the way I see the world, how to have a sceptical mind and not only how but why one should question.
And also, it shows to me that, if you don't have a answer for something, doesn't mean that it can not be true. It's just that you don't know. And for a lot of things, this is the correct and only answer that we can have now.