For many of us who like to play with mechanics, robotics, prototyping 3D-Printing and other maker/hacker hobbyists, Leonardo Da Vinci is an inspiring figure that you can use for ideas for projects or just like reading and looking at his impressive technical drawings.
The Spanish National Library Published the Madrid Codices from Leonardo Da Vinci on-line in a very nice interactive format. The topics discussed in these 2 volumes include mechanics, statics, geometry and construction of fortifications among others.
These Manuscripts have been discovered quite recently in 1964 (after being lost for about 150 years), and you can now access them for free on the website of the Spanish National library, http://www.bne.es.
The Interactive format they have used adds some really nice functionality to them. You can mirror the pages (handy because Leonardo Da Vinci used to write mirrored) and adds citations and transcriptions for parts that are unreadable or very hard to read and open animations of some of the designs in action.
Access the codices here: http://leonardo.bne.es/