Below are Newton's main works. He did not like to publish, but when he did, they were filled with information! A true math/science geek will give these a try. The rest of us may have to live with simply understanding gravity and force. You decide which side of that fence you want to be on.

• Newton, Isaac. The Principia. Berkeley: University of California Press, 1999.
  (Subtitle: Mathematical Principals of Natural Philosophy. A new translation by I Bernard Cohen and Anne Whitman. Preceded by A Guide to Newton's Principia by I Bernard Cohen.)
  • I say skip the introductory guide and go straight to the Principia. There are three books in the Principia, preceded by well-explained definitions. The first book begins with Newton's derivation of finding the area under a curve, and deals a lot with math and definitions. The second book deals with his laws of motions (including...gravity!!!). The third book deals with the specific motions of each planet, an anti-climax for the math/physics buffs. The first two books read like a textbook (which they really are), but are from the source, and such clear writing is a treat in a genius. The math is well laid out with visuals, examples, and proofs. The physics is explained in detail. An ambitious book for those already hooked on math/physics, but it is simply brilliant. The math may look different from what you are used to. See if you can get it all.
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• Newton, Sir Isaac. Opticks. New York: Dover, 1952.
  • I suggest not missing this volume, as there is a tiny two-page forward by Einstein, who quite prettily puts it to the reader that this is a delightful book of leisure...and you should read it to appreciate nature and essentially enjoy life. Hahahahaha! But seriously, only read this if you like the topic and want to learn more than your basic education offered!