You can prove the Pythagorean Theorem for yourself, using the proof described on the Pythagorean Theorem page. Just get a square piece of paper, and draw the lines on it that you see in the third drawing (the drawing of a square). Draw the diagonal lines as in the second drawing, and then cut the pieces up and rearrange them as in the third drawing.
Or you can just print the drawing on this page and cut up the drawings with a pair of scissors after you have printed them. You might want to blow the drawing up bigger to make it easier to work with.
Okay, now that you’ve done that, check out Archimedes’ proof of the formula for the circumference of a circle.
What’s Your Angle, Pythagoras? A Math Adventure, by Julie Ellis and Phyllis Hornung (2004). For teens.
Pythagoras and the Pythagoreans: A Brief History, by Charles Kahn (2001).
History of Greek Mathematics: From Aristarchus to Diophantus, by Thomas L. Heath (1921, reprinted 1981). A lot of Euclid, but also describes who the other major Greek mathematicians were and what they did.
Episodes from the Early History of Mathematics, by Asger Aaboe (1997).