Reading a book on a Kindle sends Amazon a lot of data about reading habits. How fast pages are turned, font sizes and views, and device details.
The Kindle sends device information, usage metadata, and details about every interaction with the device (or app) while it’s being used. All of this is linked directly to the reader account.
Opening the app, reading a book, flipping through a few pages, then closing the book sends over 100 requests to Amazon servers.
Full article: Kindle Collects a Surprisingly Large Amount of Data