I'll like to recomend reading the book by Bernard Schutz, 2003, "Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity", Cambridge U. Press, 462 pp. ISBN: 0 521 45506 5. O.F.Linn QC178.S36 2003.
As the title indicates this is a good introductory book to a very important subject in modern science. Bernard Schutz using a clear and direct language allows readers that are not specialists understand and most importantly enjoy the discussion of basic scientific ideas. The understanding of “Gravity” as it is related to the functioning of the universe is of paramount importance for the understanding of all natural phenomena. Schutz gives examples from astronomy to show how the universe can be used as a laboratory and how measuring distances and masses, and calculating their ratios, is used to infer laws and deduce the existence of black holes. Other astronomical objects and phenomena, such as neutron stars and cosmic rays, are mentioned as indicators of the kind of knowledge we can gather for the understanding of how the universe functions. Some one who is interested in understanding the principles of the time-space relationship and the correlation of mass and energy should read this book.