Works from 1916 and before are public domain? Not a bad start.

It is a really great to read the beautiful color version of the *First Six Books of the Elements of Euclid" online for free. https://archive.org/details/firstsixbooksofe00byrn

Meanwhile it is selling on Amazon for up to $100 http://www.amazon.com/Oliver-Byrne-Six-Books-Euclid/dp/38365...