This is the great man's attempt to explain the two theories to the lay person in 1916. The treatment of Special Relativity is more or less complete and not hard to follow.
Aside: No real math is involved in the book. But a good physical imagination does not hurt.
The treatment of the General Theory is quite cursory, mostly because the General Theory is much, much more complex that the Special Theory.
I have read such explanations many times. It was interesting to read them in Einstein's own (translated) words.
The only point of interest for me is noting that, in 1916, the metaphor for rapid travel was railroads. When it came to freefall (zero-g), there were no comparators. Today, the idea of weightlessness is common, and a spaceship is the usual vehicle for motion related metaphors.
Einstein used basic physics, and a preternatural ability to cut to the bone of an argument, to take the fact that the speed of light is constant regardless of you motion and turn it into a simple paper that was not fully accepted by his fellow physicists for another decade.
Lee Moller is a life-long skeptic and atheist and the author of The God Con.