That poor book. But that's a good idea! they can always feed new pages in. At a later point, Dorian might want to recover the book (or possibly send the runes somehow?) and add a second set of runes to the books that will let them set it up like his rings so that, before the messages appear, the user has to do an activation thing. Just to stop anyone from opening up a book and reading a message while Waver or Dorian are looking away.