Before reading this book, I had a rather confused idea about relativity. Basic concepts lying around in my head without anything to connect them in a coherent way. I knew that time slows down the closer you get to the speed of light. I knew that the same happened the closer you got to a black hole. I had no idea how those things were connected. This book helped a lot putting some order in my head.
There's no math here at all. It begins with a couple of assumptions (e.g.: everyone records the same speed of light) and, using a series of thought experiments, extrapolates the basic concepts of special and general relativity. I just love this way of doing things, because once I understand how something works, then I tend not to forget about it. Not the same if you just throw sparse notions at me.
The only downside is that now I've been hit by a huge fascination for black holes. I think my next read will be about that (if I can find something not too technical).