In the early 1930's general relativity was a fairly new subject, and must have seemed like the domain of just a few. With its clear presentation of the subject Relativity, thermodynamics and cosmology
by Richard C Tolman would have made the field accessible to a much larger number of students. At the time it was written the expansion of the universe had just been discovered. A closed universe was the favoured model, but Tolman warns against choices based on wishful thinking. This book will provide a valuable resource for those interested in the development of the early stages of today's cosmology.
But I feel that the book isn't just of historical interest. Tolman's approach is to take each of mechanics (including continuous media), electrodynamics, and thermodynamics and show how to derive the special relativistic form of the equations. For each case he demonstrates how these equations may be transformed into general relativistic versions, and finally gives a compact, tensorial form of the equation. More modern treatments of general relativity are more likely to focus on particular areas of the subject, rather than giving this wider treatment of the basics, and I think that the book might have some use even to today's students of GR.