[Fat Abe]

The jpeg route allows one to insert a number of telltales, and that is the most inconspicuous, unless the thief had two copies of the book. That still seems the most preferable for an average person.

I'm not that familiar with the spec on the opf file, but can't one slip in a block like <xyz><item id="abcd1234" /></xyz>, where xyz can be any string other than metadata, manifest and the other reserved names? Or just obfuscate by using a known name type where assignment of the id does not conflict with anything else. id is used to hold the hex value of the registered user.

As we come up with new identity embedding methods, one can be sure the pirates are working on stripping programs to automatically eradicate all ownership tags, including the ISBN.