One can take those numbers a step further and WAG they are selling about 30M gadgets and 500M ebooks a year. With an attach rate of about 16 books per device.

(Hey, this is a game anybody can play!)
Edit: Oh and if you believe the margin numbers, ebooks are subsidizing the *rest* of Amazon.
Doesn't mean I believe them, but they are certainly more credible than the whines about Amazon "selling ebooks at a loss".