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.
I read this book at a time when I had lost my religion which had been very dear to me and the foundation all of my thinking. I was reading lots of different religious texts. This was the first thing I read that said that how we know what we know is more important than what we know. It was the first philosophy I had encountered that would challenge even evidence in favor of itself on the grounds that that evidence didn't meet a certain bar of quality. That was huge and this book helped me understand that concept. Which really made scientific skepticism stand out from other belief systems.