Hi Paul,
I dont believe any of the current available systems will completely secure your ebooks. Its much the same with shareware, eventually someone cracks it and its then freely available to everyone.
All you can do, just like Kjell has stated (I think), is use a method which will make it more difficult to copy, but if a waster is intent on reselling the ebooks there is little a limit on the number times it can be opened with do to deter them
There is another method you could try which packs the document within an .exe file, a client of ours uses it. If you are interested let me know and I will ask him for details.
Dave.