Subject
6 free books about gravitation, free to read online. Chapter 1 of any book is free, no account needed.
Aether and Gravitation
William George Hooper
Einstein and the universe: A popular exposition of the famous theory
Charles Nordmann
From Newton to Einstein: Changing Conceptions of the Universe
Benjamin Harrow
The foundations of Einstein's theory of gravitation
Erwin Freundlich
Einstein et l'univers: Une lueur dans le mystère des choses
A simplified presentation of Einstein's unified field equations
Tullio Levi-Civita
