Aava Library - Important books, for free
A collection of important literary works. Updated regularly.
Relativity: The Special and General Theory
Physics - Albert Einstein's important book on relativity, 1916.
From Newton to Einstein: Changing Conceptions of the Universe
Physics - Benjamin Harrow, 1920. With articles by Prof. Einstein.
On the various forces of nature and their relations to each other
Physics - Michael Faraday, 1791-1867 (edited by William Crookes)