To mark the 100th anniversary of Einstein's general relativity the Minkowski Institute Press publishes the first English translation of a very rare book on general relativity (its only Russian publication was in 1924), which turned out to be the last book by A. A. Friedmann (co-authored with V. K. Frederiks). This is the first and the only published volume of a five-volume book project on the foundations of the theory of relativity, brutally terminated by the untimely and tragic death of Friedmann on 16 September 1925.Despite the fact that this book was published in 1924 and despite the presence of some unconventional notions and notations in it, this is still a valuable book, because it is written by two deep thinkers, particularly Friedmann who in 1922 had the deepest understanding of the cosmological implications of Einstein's general relativity when he first showed that the Universe may expand (which was later discovered by Hubble). What also makes this book valuable is that Frederiks and Friedmann develop the formalism of tensor calculus from a physical point of view by showing why the ideas of general relativity need that formalism. In this sense the book can be even used for self-study.