Originally Posted by cybmole
PPS my previous question about "pottermore" style watermaking was not "is it secure" but "how easy is it for an author to do"
Sorry to be part of the deluge which inundated your request
The coding itself might be very simple, provided that you don't demand that it be very difficult to remove. Something like adding a personalised line to the legal page could be done by a simple script which searches for placeholders in your file and replaces them with the customer's name, for instance. You could also add steganography to images, encode stuff in whitespace, and do all sorts of tricks which might be more difficult to remove (but never impossible).
The problem is that this necessarily needs to be done at the point of sale (download), which likely means that *you* would have to administer customer sales/support, and probably all infrastructure like payment systems, servers, security, and so on. This is fairly complex, and probably not feasible to do for a private individual. That is where distributors come in, they can usually handle watermarking as well as technical DRM, but at a price.