Always Fresh Books
Books, even ebooks are fixed things. What you get is a finite product of that edition. Most likely by the time you read it, at least some things have changed.
Create a format that allows the author to push new editions to current buyers of previous versions. The reader will get an alert that he has now a new edition; a brief description of the changes and additions; and the possibility to read any of the editions published, and at any point also see the current text’s other edition versions.