For this invention will produce forgetfulness in the minds of those who learn to use it, because they will not practice their memory. Their trust in writing, produced by external characters which are no part of themselvesSocrates to Plato in Phaedrus
"A new study which found that readers using a Kindle were "significantly" worse than paperback readers at recalling when events occurred in a mystery story is part of major new Europe-wide research looking at the impact of digitisation on the reading experience." ... "The Kindle readers performed significantly worse on the plot reconstruction measure, ie, when they were asked to place 14 events in the correct order."
The researchers suggest that "the haptic and tactile feedback of a Kindle does not provide the same support for mental reconstruction of a story as a print pocket book does".
"When you read on paper you can sense with your fingers a pile of pages on the left growing, and shrinking on the right," said Mangen. "You have the tactile sense of progress, in addition to the visual ... [The differences for Kindle readers] might have something to do with the fact that the fixity of a text on paper, and this very gradual unfolding of paper as you progress through a story, is some kind of sensory offload, supporting the visual sense of progress when you're reading. Perhaps this somehow aids the reader, providing more fixity and solidity to the reader's sense of unfolding and progress of the text, and hence the story."
From the Guardian If haptic and tactile feedback to give a sense of progress in a book is so important how could this be added to ebook readers? One possibility is a weight that moves from one side of the ebook reader to another as you progress. This would mirror the feeling of a book starting heavier on the right and ending heavier on the left. Assuming you are reading English and not Manga or other back to front based moving system. This could be accomplished with by moving a marble as the book progresses.
Another option is to change the weight based on how far through the book you are. By adding Pez dispenser to the back of the ebook reader that dispensed sweets at points during the book a physical change in the book would result.
Once you are doling out sweets anyway you could flavour them based on the book in question. That would mean you could supply a flavour for the text at certain points which would provide the sort of physical feedback books currently do not. What flavours would go with which books and where? I have never been able to get access to the kindle SDK. But this seems like the sort of hardware project you could try at a hack weekend if you had access to the kindle api.