Something that had confused me about your proposal is starting to become clear. So just to clarify, under your scheme someone who paid for the book can give it to whoever they want but anyone who received it can't pass it on - is that correct?
Because I think it might be a bit confusing. On the one hand you're encouraging lending/sharing on the basis that that's like paper books, but you have to educate people that the lending/sharing can only go one level - unlike paper books.
But it makes more sense of talking about being able to do "whatever you want" and still have "illegal distribution".