I recently bought a Craig 738b Android tablet (NOT recommended, btw, although the display is nice), and had my first experience with e-books, these from Project Gutenberg (no images?). Wanted to "go horizontal" and explore what I'd downloaded. Discovered something unexpected. Lying on my side, I found that the accelerometer (gravity sensor) kept making the display rotate to an awkward angle. I needed a disable function.
No, we'll never have a gravity-sensitive paperback.