What are the minimum reading requirements for understanding Newton's Principia Mathematica Naturalis Philosophae? For a long time, the self-contained nature of Newton's Principia has intrigued me. At a glance, it looks as if Euclid's Elements would be the only required reading for understanding his arguments. But it's still pretty tough going. Are there any lesser-known works from his time or before his time (I'm not looking for something to explain him to me, I want to read him and understand his arguments from first principles, the way he wrote them) that might have been obvious points of reference for people at the time he published, that simply haven't survived the way Euclid has?

You should start with Euclid's Elements, then Apollonius' Conics' and Archimedes' Works. All these works have been edited by T.L.Heath. Kepler and Galileo would follow. Some first year physics and mathematics texts would help. And lab work in  first year college physics course to appreciate the experimental philosophy. Watch all 52 episodes of The Mechanical Universe. 
