I have written a short guidebook that I have been selling hardcopy. There is now interest in a digital version. How do I protect this guidebook so only the people who purchase it have access to it? I don't want them to be able to forward it to someone or load onto more than one computer?
In general, this is impossible. Anything your users can access, they can create an insecure copy of.
That said, this is a problem that people have tried to solve before, and they have come up with some partially effective methods (see iBook or Amazon Kindle, as vcsjones suggested in a comment). These methods rely in part on punishing violators, so that copying the document, while nowhere near impossible, is difficult and unappealing.