Quite unfortunately, in this overall well written book the author makes some serious mistakes regarding the essence of the GR. For instance:

1. On the page 10, the author states that in non-inertial
frames the following postulates apply:

"G1. The laws of nature are the same in all reference frames."

"G2. An observer with arbitrary motion may consider himself
to be at rest and the environment as moving."

The both statements, especially G2, are incorrect. An observer
in an accelerated frame definitely can tell that he is
in an accelerated frame, because, for instance, he will be
feeling g-forces, the period of a pendulum will be changing
from the magnitude of the acceleration, etc., quite different
from an inertial (non-accelerated) frame, where everything
will be floating in weightlessness! The correct statement,
describing the equivalence principle is:

"An observer in an uniformly accelerated frame may consider
himself to be at rest in a frame with an uniform gravitational
field. The laws of nature are the same in the both frames."

Since "uniform" gravitational fields do not exist in the
nature (except as an approximation in a very small volume),
Einstein came to the concept of space-time curvature expressing
the "real" gravitational field, etc.

Because of this initial error, on the page 33 the author,
when explaining the (in)famous "twin paradox" concludes
"In order to arrive at a clear answer to these questions,
we shall have to use the result from the general theory
of relativity." This statement is absolutely false, as it
had been shown countless times before in various texts, as
long as the "real" gravitational field is not present
(the Riemann tensor equal to 0), the twin paradox can be
easily resolved using only the special theory of relativity
(see the book by Taylor and Wheeler)! This argument has already
been discussed to death, so it's quite bad to open it anew
in this book :-(