Albert Einstein is rightly considered one of the greatest scientists of all time, and his two theories of relativity - special and general - are the crowning glory of his scientific oeuvre. They have fundamentally reshaped our thinking of the most fundamental concepts - space, time and matter. These two theories have also withstood the test of time, and a century after they had been formulated they are still almost entirely used in their original formulations.
H. A. Lorentz was a distinguished physicist in his own right, and one of Einstein's closest scientific and personal friends. The special kind of the coordinate transformations that characterize the special relativity have been named after him, and he is one of the first people to whom Einstein described his general theory of relativity. In that regard he is certainly one of the foremost early authorities on the subject.
This short book primarily deals with the general theory of relativity. It was written shortly after one of the most startling predictions of the general relativity - the deflection of light by the sun - was confirmed by the British astronomer Eddington. The public was immensely fascinated by this incredible phenomenon, and there was a need for an accessible and informative explanation of general relativity. Unfortunately, even though general relativity is an incredibly "beautiful" theory in its own right, the mathematical apparatus required for its full understanding is formidable. This short introduction completely sidesteps all mathematical language and presents the subject in terms of the most fundamental concepts.
It is quite remarkable that a short popular book like this one has withstood the test of time. As a college physics professor who works with general relativity I could not think of much that I would add or subtract from this book. However, this is a rather short book and if a reader would like a bit more information on the subject that is still at the level of general reader I would strongly recommend Relativity A Very Short Introduction.