What if someone got the rights to a bunch of different popular books and inserted the same extra into each book? No one would know where in the book the extra is, and perhaps they wouldn’t even know which books have the extra in it! It would become a little “Easter egg” for those who found the “Hidden Extra” in their book, especially since not all the books would necessarily have it (if it was inserted after the book became popular). Eventually, maybe the hidden extra guy could create a story that somehow links all the books and hidden extras together. Also, who knows? Someone might have already done this, but hasn’t revealed it yet. Brandon Sanderson has started something like this, inserting the (extra) character Hoid into at least three of his different book series, and (I hope) is planning to explain how they connect. As cool as it may be for Sanderson to be connecting his own books’ universes together, imagine how awesome it would be to have the universes of the books of several different authors all connected!