Online value-add for books 

Tech books and textbooks have online errata and addenda. Why not other kinds of books? You've bought the paper book, audiobook or ebook, so you get a chit to download a subsequent bonus: JK Rowling's musings on further adventures of the Harry Potter characters; Carl Bernstein's analysis of the campaign as a follow-up to the biography of Hillary that came out before the contest began in earnest.