Something worth considering for print books are QR codes on the cover to provide easy access to the book’s web page on a mobile phone or tablet. Ubuntu has ‘qrencode’ which takes a URL and generates a .png file of the relevant image, Windows must have something similar.
qrencode -o emg.png -s 6 http://www.edwardmgrant.com/tartarus.shtml
I’m going add them to the new Createspace cover and see how well they work. At 300dpi they may need to be a bit larger to be easily readable, but the web consensus seems to be not to go much smaller than half an inch, and these would be about 0.75″.