Robert M. Wald is more known by his (very technical) book "General Relativity", where he explains Einstein's theory using a somewhat (sometimes too much) hard mathematical description. The main problem with this book, "Space, Time and Gravity" seems to be, for me, also its hardness; it is a clear and well written book, but maybe with language and focus some steps too high for the general public. Let give me an example: the book has ten chapters; the three first ones give a beautiful logical description of how space and time are viewed in Physics, but the next chapter becomes a bit too complicated, having a simple description of the Singularity Theorem, which for me seems a technical matter not very appealing. The final five chapters give an interesting account of the theory of black holes, but again this account seems to lack some taste, reminding me of a breakfast made of a superb toast served without jam or butter or anything to drink... However, I would recommend this book for undergraduate students of physics. For readers with a not-so-good mathematical background I would also suggest "Flat and Curved Space-Times" by G.F.R. Ellis and R.M. Williams (unhappily out of stock). The general public probably would enjoy more the reading of Einstein's "Relativity : The Special and the General Theory" (Paperback - May 1995) (a very recommendable book!) or the lengthy "Black Holes and Time Warps : Einstein's Outrageous Legacy", by Kip S. Thorne, et al. (Paperback - January 1995).