Carl Sagan's "The Demon-Haunted World: Science as a Candle in the Dark" http://en.wikipedia.org/wiki/The_Demon-Haunted_World Also perhaps Bill Bryon's "A Short History of Nearly Everything" for an entertaining broad view of a variety of scientific areas: http://en.wikipedia.org/wiki/A_Short_History_of_Nearly_Every...
The Demon-Haunted World was incredibly formative for me, and I was around 14 when I first read it. Prior to reading it, I believed wholeheartedly that UFOs were alien spaceships, and despite already being fairly certain I was an atheist, I was still on the fence about ghosts. The most important thing about that book was that it didn't teach me that things I believed weren't true because a distinguished scientist said…
That book is one of the only gifts I can remember recieiving when I was a teenager, and I still have it on my shelf. It is fair to say that it can be a life changer.