> Is there any data released on ads Vs no ads versions?
Do they offer a tracking vs no tracking option too? The absence of adverts does not mean the absence of tracking.
Even if you E2E-encrypt each user's data for cloud storage and have devices join a P2P-keybag, ala iMessage, consider the ad-tech department of your same company as if they were an external adversary for a moment. What would an external adversary do in that situation? Traffic analysis of updates to the cloud-side E2E-encrypted bundle. That alone would still be enough to create a useful advertising profile of the customer's reading habits, since your app is single-purpose — the only reason for that encrypted bundle to be updated, is if the user is flipping pages!
And, together with the fact that your ad-tech department also knows what books the customer is reading (because your device can only be used to read books you sell, and thus books you have transaction records for selling them), this department can probably guess what the user is reading anyway. No matter how much your hardware-product department tries to hide it.
Sure it can. You encrypt the data so the client can read it and the server can't. When you add a new device, you e.g. scan a QR code on your old device so that it can add the decryption key to the new device, and the server never knows what it is.
In this regard, Amazon is among the worst of the big tech companies that I interact with. Google lets me turn off personalization—when I go to Youtube, I see an extremely generic set of recommended videos. But I can’t do it on Amazon.
(I don’t use Facebook, not sure what can be switched off.)