The booklet was written by an eminent scientist in 1920, using the style of language of that era, with some long sentences that science writers of the time were prone to use. Conveniently, some words used, e.g. "perihelion" can be looked up in the dictionary that is free with the Kindle.
It was written only a year after the first experimental confirmation of Einstein's theories of relativity and so when Lorentz wrote "If his theory is correct as it stands, there ought, in a gravitational field, to be a displacement of the lines of the spectrum towards the red. No such effect has been discovered" he should probably have added the word "yet". In the more than 90 years since the book was published, this movement of the spectral lines due to gravity has been confirmed.
The book introduces you to Einstein himself and some of the scientific questions of his time, Einstein's radically different approach to them, a summary of the predicted effects of relativity and then some of the experiments that had been performed to confirm them. Non-mathematicians can rest assured there is not a single equation in the book.
Those familiar with accurately plotting a position will be familiar with degrees (°), minutes (') and seconds (") of arc where 1 second of arc is one 3,600th of a degree and thus a VERY small angle. Seconds of arc are referred to several times, but one typo uses ' instead of ". However, it is obvious from the context that Lorentz meant these very tiny angles called seconds rather than minutes of arc. I don't feel this detracts from the book at all.
I read the book at a single sitting and since it now 8:30 p.m. and I haven't yet had dinner, in my opinion it makes a good read!