sorry, that reply was to
"is there a book that by consulting the original sources (articles, letters etc etc) traces the evolution of Einstein's thought that lead him from the Equivalence Principle to postulate General Relativity as curved space."
short answer: probably, you mean like an...
I would of said true "distance" is flat, but due to motion and the deformity of apparent position as opposed to true position it is often modeled as curved, its important to remember the difference between a mathematical model to simulate the effect and reality.
Quick short answer your probably looking for: Think of a prism... think of the sky, think of a lens... the effect of moving from one medium to another medium of different density distorts the passage of light, effects we see as magnification, refraction, diffusion.
I don't know much else on...