Haptic feedback is still an area where real books win over e-book readers such as Kindles. Being able to tell how far through a book you are by the feel of it, by the balance and thickness of the pages adds something instinctive to the reading process.
My idea is to have a little linear actuator with a small weight on it that spans the width of the ebook device. Just a very small motor (the type you get in phone vibrators) and a small worm-gear (like you get in floppy disk drives) would do, and wouldn’t take up much space. The weight would gradually move across from left to right as you moved through the book. This would allow you to assess your progress through whatever tome you were currently reading just by feel.