Bertrand Russell wrote the 'ABC of relativity' in 1925. Now, about 90 years later, I read it. These are two examples of events in spacetime, and events is what relativity deals with. It tells us that the distance and duration between two such events as measured by differently moving observers can be different, and there is no reason to prefer one observer's version over the other. Nevertheless, there exists an entity called 'interval' that each can measure and will agree upon. So not everything is relative.
Popular expositions of relativity abound, so what is there to recommend the present one? First, it's Russell. You can expect both clarity and depth. Russell goes beyond matter curving spacetime and clocks running slow for someone who is moving. He tells why Newton's law of gravitation is not compatible with relativity (if different observers measure distance differently, they will also measure the gravitational force between two objects differently). He explains how the very notion of a gravitational force emerged due to influence of Euclidean geometry (if an object is not moving in a Euclidean straight line e.g. earth around the sun, there must be some influence to explain this behavior), while relativity does away with gravitational force as something exerted from afar to explain motions of objects as responses to the local features of spacetime. And how, the very notion of matter as which persists through all of time and is not present at more than one place at a time is dependent on complete separation of space and time, and hence needs revision in the era of relativity. And how scientific convention and limitations of our senses can influence our theories (in the Michelson-Morely experiment, do you say the length changes or speed of light varies?). And so on.
It would be foolish for me claim to have imbibed everything the book has to say. But I can certainly say this. It was a refreshing read despite having met popular accounts of relativity before. It is a popular account, so you will certainly learn something. And then of course, it's Russell.