Google, Amazon, Project Guttenburg is giving me trouble.

They don't or can't find copies of these books: (Hopefully translated to English)

Johannes Kepler
- Harmonices Mundi
- Astronomia nova

Isaac Newton
-Mathematical Principles of Natural Philosophy

Galileo Galilei
- Dialogue Concerning the Two Chief World Systems

Nicolaus Copernicus
-On the Revolutions of the Celestial Spheres