What's Your Angle, Pythagoras? A Math Adventure - Julie  Ellis, Phyllis Hornung
A fictional account of how Pythagoras developed his famous Pythagorean theorem. It's interesting because the story, text and pictures seem intended to target a very young audience, but the math presented will likely be well over their heads. Still, it's a fine book in many ways - attractive illustrations with clear explanations of how the theorem works.