Originally Posted by Jellby
You could encrypt the ePub as any other file (zip with a password, or similar). Then the end user would have to unencrypt it. But once this is done, the unencrypted file can be copied, shared and viewed unrestrictedly...
But, even if it was encrypted and the person wanted to give it out, all that would need be done is give the key with the copy and there you go.
There is no foolproof method to make this eBook secure.