Measurement by Paul Lockhart, a book on geometry for those who have little background in mathematics. I have not actually read it, but I have heard it hyped quite a bit by people who know what they're talking about, and I'm quite prepared to believe that it is excellent given that it was written by the author of A Mathematician's Lament. If you are already a math expert, you might not get much out of this, but if you want to become a math expert, I recommend it.