I really like N. M. J. Woodhouse's two books on relativity: General Relativity (Springer Undergraduate Mathematics Series) and Special Relativity (Springer Undergraduate Mathematics Series) (Volume 0). They're both short, well organized, exceptionally clear and mathematically impeccable. Woodhouse is a very gifted writer: he knows how to get to the point!
Even though the book is short and mathematically oriented, it does not shirk physics motivation, as you can see via the Look Inside this Book function. Woodhouse manages to cover the basic or essential aspects of many key topics in just 178 pp. For example, Einstein's Equation is introduced and discussed in chapter 6 (pp. 89 - 94); the curvature tensor is discussed on pp. 96-98 and Killing Vectors on pp. 102-104. Other chapters deal with Spherical Symmetry, Orbits in Schwarzschild Space-time, Black Holes, Rotating Bodies, Gravitational Waves, Redshift.
Fast pace, yes; but he makes every word, definition, equation and theorem count! Anyone, like me, who has been frustrated by the many introductory general relativity books that are either confusing -- mathematically or otherwise -- or else just too detailed (I love GR but am not gonna become a physicist :), should find this new (2007) introductory book a "breath of fresh air".
Be forewarned though that they're written for senior math majors and so not suitable for anyone whose math skills are sub-par. But if your math skills are honed, both books are really a pleasure to read, especially compared to the mathematically inadequate, long-winded and sometimes downright confusing expositions of many other books.
Also, in some places I think the book is too terse, and because of this, is best used as a supplement to a book providing more complete details on certain key topics, e.g., Christoffel symbols, parallel transport, covariant derivatives, and Riemann tensor (curvature tensor).
Because of these limitations, I have taken away one star, hence the 4 star total.
[If your special relativity isn't up to par, I also recommend his book Special Relativity (Springer Undergraduate Mathematics Series) (Volume 0) as a "prequel".]