Just jumping in on this one. Software like Natata should be frogmarched to the village square and pelted with rotten fruit until it apologises.
"The only issue is permission and rights. You want only the people who purchase it to be able to download and open the book, sometimes also limited to only one physical machine."
You're not important enough to make this worth giving even the faintest damn about. I say this with absolute certainty, because I can think of absolutely nobody who is important enough to get away with the restrictions that commercial ebooks hand out as a matter of course.
If you approach it from the angle of 'How can I stop people reading my book without permission?', you'll waste loads of time on a futile quest for technology that will annoy your customers and slow down passing pirates for about ten seconds, max. If I've bought your book, there is not one single thing that my printing a copy out to read on the train, or reading the thing on my desktop and PDA, is going to do to affect your profit, because the chance of my buying two copies is precisely zero. Possibly the person you're trying to get will get stopped at the gate, but that won't make you any more money. However, by annoying me, a paying customer, over something that makes such little difference, you've just shot yourself in the foot and pretty much guaranteed losing my custom on future books.