I’d be interested in seeing how ebook sales trends match up with the shift from epaper to OLED/LCD displays on ebook readers. After all, the first few generations of Kindles and Nooks had epaper display — which in addition to having good battery life, mimics the properties of a paper book quite well; current generations (particularly post-ipad) have luminescent displays with color and faster refresh rates and are essentially just tablets. E-paper displays, because they lack back-lighting, are ideal for before-bed reading; because of their slow refresh rate, they aren’t ideal for anything *other* than reading full pages of text. By introducing tablet-style backlit displays, the new dedicated e-readers may be driving people who would otherwise use them for before-bed reading back to paper, while pushing the rest of their users toward non-book content.
(I’m also wondering if these trends you’ve mentioned extend outside of the Amazon-and-BN for-profit-ebook universe. Do the same trends exist in, say, download rates for epub files on Project Gutenberg? What about archive.org, or the pirate bay? It may be that prices or circumstances are lowering the rates at which people download ebooks that they need to pay for — and a variety of things could cause this, ranging from DRM policies to perceived reliability of the devices. After all, if you own a Kindle and all the books you own exist only on it with no capacity for backup, then bricking it would mean losing your entire library; it then makes more sense to buy the books on paper or to pirate them, since in the former circumstance it’s more difficult to lose all of them and in the latter circumstance it’s not difficult to back them up or get them again later.)