We're probably near a point where everyone who wants to own an e-ink reader already has one. At that point, these folks are buying at least some of their books

We're probably near a point where everyone who wants to own an e-ink reader already has one. At that point, these folks are buying at least some of their books for and through the device (I wonder if this retailer gets a cut of that), but they're not buying new devices very often. So it makes sense that sales of the devices would plateau after a while.