A privilege to temporarily view the content on devices that the seller approves, as long as they approve. This privilege can be revoked at any time, including for reasons having nothing to do with you (such as the seller having a fight with the copyright holder), and the content can be modified or removed at any moment. And you pay for that more than for the paper copy, because it sucks to be a sucker.
Nothing. Or perhaps "buying the ability to access the book, where the provider can alter/censor/revoke any time without prior notice" if we really want to use the word "buy".
