Or in any e-book reader, for that matter. And here’s why: The quagmire of DRM and proprietary file formats.
I have quite a few books on my bookshelves that are more than 20 years old. Some are even older than that. A few were printed before I was even born. Five years from now, will any file formatted for the Kindle be readable anymore, on any system? Possibly, if the product does well enough.
But what about 7 years from now? Or 10? Or more? Not so likely.