The theory of relativity, (The international series of monographs on physics)

In spite of being an old textbook, published originally in 1952, this is one of the best introductory textbooks to both special and general relativity. It is far superior than many textbooks on the topic that have been published later. This is one of the five textbooks on special relativity cited by Steven Weinberg in his classic "Gravitation and Cosmology". The author achieves a balance between clarity and completness that is missing in many other textbooks. For instance equations for the Lorentz transformations, velocity and force transformations are given in tridimensional notation and proved in simple terms. I recommend this textbook as one of the best introductions to Special Relativity. On the other hand, the part of the book discussing general relativity is a little outdated even tough correct.