Albert Einstein finished up his General Theory of Relativity in November, 1915, 100 years ago. Because we use Base 10, this is significant.
General Relativity ties together curvature in spacetime with the energy and momentum of matter and radiation. This has a lot to do with gravity. Einstein himself wrote the book on General Relativity, but it has been covered in a lot of other places as well, including a recent treatment by historian of science Tom Levenson, The Hunt for Vulcan: … And How Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe.
Nature (the journal, not the big thing we all commune with like it or not) has a special production to celebrate this important anniversary, including an eBook produced by Scientific American, various commentaries, and features. You might want to check that out. But of course, if you really want to understand what Einstein was saying, there are few works better than the classic Mr Tompkins, by George Gamow.
Two books published just to celebrate the anniversary are General Relativity: The most beautiful of theories (de Gruyter Studies in Mathematical Physics) and General Relativity and Gravitation: A Centennial Perspective, both highly technical and very expensive.