This book, so careful in its proofs, seems inaccessible due half to the revolutionary nature of the insights and half to the manner it is presented in.
It explains simply enough that gravity is not a force (as Newton had supposed), but merely a set of conditions that vary with your point of view. The key to point of view - as Tim Leary pointed out in his early work on "social phsyics" - is how fast one goes.
It speculates on the paradox that the universe can be finite but also without a practical boundary or physical end-point.
Most grandly, it predicts how motion works in the universe on the largest scales. This is backed up by the red shift of the sun, discrepancies in the orbit of Mercury, and an experiment from earth in 1819 which established his reputation with exact and prophetic matches.
I appreciate how carefully he reaches beyond non-Euclidean geometry. I like the care he takes to back up his claims too. But what I do not like much is the analogy of the man in the box being hauled upward by a rope. This is supposed to illustrate the relative nature of gravity: for the man, gravity exists while he moved up in the box; and for an observer it is a different gravity effected by different conditions. Are you confused yet? I certainly was, and perhaps we can be glad Herr Einstein is not here to explain it with an even less accessible analogy!