The University of Cambridge, as part of their Digital Library project, have digitised over 4000 pages of Isaac Newton’s writings and made them freely available on the web. Available works include some old college notebooks, his rough work from when he developed his theories of calculus, and the mighty Philosophiæ Naturalis Principia Mathematica. The Principia (as it’s more commonly known) was his crowning glory, in which he states and proves his Laws of Motion and his theory of Universal Gravitation. Although the three-part book was deliberately written to be difficult to read (He valued his privacy, and wanted to minimise the number of people who would talk to him about his work), it’s revolutionary ideas made him so famous that he is still a household name more than three hundred years later.
Cambridge’s collection of Newton’s books were all originally his own personal copies. The library has all his writings on science and mathematics, although his other works on theology, alchemy and chronology were all distributed to other collections a long time ago.