Sidelights on Relativity by Albert Einstein (1879 - 1955)

Sidelights on Relativity contains ETHER AND THE THEORY OF RELATIVITY, an address delivered on May 5th, 1920, in the University of Leyden; and GEOMETRY AND EXPERIENCE, an expanded form of an address to the Prussian Academy of Sciences in Berlin on January 27th, 1921. (Intro from Project Gutenberg)