Even if Amazon sends logs back to their servers on book titles, etc. there is no reason for it to be encrypted. What possible harm to you could come from someone intercepting a list of the books you've read? Assuming that someone could intercept such a log.
I know that they can pull a log from the Kindle if you are talking to Kindle Support about a problem, but I don't know what all is in it. Nothing I would worry about them knowing, anyway.