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There are only a few methods available. It all depends on what you are willing to pay and readability of the book. Basically there are two encryption schemes. One by Adobe and one by B&N. Almost or all e-readers can read the Adobe encrypted files, but only the nook (afaik) can read the B&N encrypted files. Both cost a substantial amount of money to buy the encryption software. Perhaps Apple has a third encryption for the iPad, but I do not know nor care.
Any other encryption/security measure will lead to incompatible epub files, because the reader software does not know how to handle it.
Both encryption schemes have been cracked though. Basically I would not buy a book which has DRM or is encrypted which is not removable. A lot of people are that way. There is also no lock on a paperback or hardback when I buy it. It is silly and will only harm those who want to buy and will not stop pirating.
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