Geared toward readers already acquainted with special relativity, this book transcends the view of theory as a working tool to answer natural questions: What is a frame of reference? What is a "law of nature"? What is the role of the "observer"? Extensive, nonscientific treatment. 1983 edition. Unabridged republication of the second edition originally published by Wesleyan University Press, Middletown, Connecticut, 1984.
General Relativity and Gravitational Waves by J. Weber Concise introduction by a pioneer in the investigation of gravitational waves explains the related Riemannian geometry and tensor calculus, conservation laws, and classic experiments. 1961 edition.
Beyond Geometry: Classic Papers from Riemann to Einstein by Peter Pesic This is the only English-language collection of these 8 accessible essays. They trace seminal ideas about the foundations of geometry that led to Einstein's general theory of relativity.