Set up a folder on a website which requires a password for access before downloading.
If you were selling on a store like the App Store, etc. it could be encrypted. Or you could pay big bucks to Adobe to be licensed to use their encryption system.
Thanks for that Steve, so forgetting encrypting the epub itself, is there a way to add a password form on page 1 in the INDD document that would restrict access to the following pages or content?
No, not possible.