This short booklet provides Einstein's insight on the role of "principles" in the development of fundamental theories in physics. A must-read for Einstein fans and serious physicists.
This article was published in Nature, in 1921, in the special issue dedicated to relativity, which included the papers by various experts in the field such as Hendrik Lorentz, Arthur Eddington, and Hermann Weyl. Einstein's paper was the first one in the issue, and it provided some historical overview of the development of the theory of relativity, as of 1921.
This short article was originally written by Albert Einstein at the request of The London Times, in 1919, and it was intended for the general audience.If you are interested in getting a general overview, and the historical context, regarding Einstein's relativity, then this is the best introduction.