Einstein's Genie: Spacetime out of the Bottle, by Graham Nerlich

This is a book that everyone interested in understanding Relativity should read. In it, Graham Nerlich argues, lucidly and with minimum technicality, that Special and General Relativity are best understood as theories about spacetime, which is a concrete entity but not a substance. (Here he is taking our concept of a substance to require a substance to be a cause.) He argues that gravity is not a field but, instead, the geometry of spacetime explains how things behave in the absence of fields, neither causing, nor, more controversially, being caused by anything.