The volume contains new publications of five works by Einstein - his valuable book Relativity: The Special and the General Theory, written for a wider audience - and four articles, which nicely complement the issues discussed in the book. The selected works contain not only Einstein's own explanations of his revolutionary contributions to fundamental physics, but also give the readers, particularly his popular book, the unique chance to be exposed to Einstein's original thinking in action. Einstein's five works included in the volume are: 1. Relativity: The Special and the General Theory. A Popular Exposition 2. "What is the Theory of Relativity," written at the request of The London Times and published on November 28, 1919 3. "Ether and the Theory of Relativity," An Address delivered on May 5th, 1920 in the University of Leyden 4. "Geometry and Experience," An expanded form of an Address to the Prussian Academy of Sciences in Berlin on January 27th, 1921 5. "A Brief Outline of the Development of the Theory of Relativity," Nature volume 106, pages 782-784 (17 February 1921) One notable issue - the concept of relativistic mass - is not discussed in Einstein's five works republished in the volume. Although he wrote in his popular book (included here, p. 36) "the inertial mass of a body is not a constant," Einstein did not discuss the concept of relativistic mass (i.e., the relativistic increase of the mass of a body as its velocity increases), which he silently abandoned after using it in his 1905 paper, where he defined longitudinal mass and transverse mass. As Einstein's unclear view of the relativistic mass might have provided some encouragement for "what has probably been the most vigorous campaign ever waged against the concept of relativistic mass," an Appendix "On Relativistic Mass" is included in this volume to address that unprecedented campaign and to help the readers make their own decision on the concept of relativistic mass.