Journey through Genius (Score 1)

I would highly recommend Journey through Genius by William Dunham.

http://www.amazon.com/Journey-through-Genius-Theorems-Mathematics/dp/014014739X

It explains pretty nicely 12 mathematical theorems, from a proof of the Pythagorean theorem to Cantor's non-denumerability of the continuum. I used this book senior year of high school (just a few years ago) and it introduced me to some really interesting theorems. I especially liked learning that there were different sizes of infinity.