These four lectures constituted an overview of his then-controversial theory of relativity. Princeton University Press made the lectures available under the title The Meaning of Relativity, the first book by Einstein to be produced by an American publisher. As subsequent editions were brought out by the Press, Einstein included new material amplifying the theory. A revised version of the appendix Relativistic Theory of the Non-Symmetric Field, added to the posthumous edition of 1956, was Einstein's last scientific paper.-- "Physics Today"