You could use some form of encryption but once unencrypted then the same problem occurs. The user can do whatever they like with the contents.
You could 'brand' the content itself by adding comments here and there within the XML files themselves before burning to CD using ""
A search should find you a utility to add some branding into the images themselves which are invisible. It's a basic form of text encryption, or hiding text within an image coding. This would probably do the job but only with jpegs http://www.safe-install.com/programs/encrypt-text-in-picture.html

Another good idea is to lodge a copy with a Solicitor as a sealed copy, in this case anyone producing a 'copy' after the date of your release is going to be on very shaky ground if claiming it is their own work. You should be able to claim rights to royalties without much hassle :)It does work I've used this myself.