IP address and location as you move from place to place (LTE/wifi).

Reading habits and library

Political leanings

Other associated metadata that may do more to link that device to your overall advertising profile and help deanonymize you for other data brokers.

Another commenter lists a few options, but even the most basic of data - what a person chooses to read - is personal and private enough to warrant caring about.

Is this not already available from your shopping data?

Assuming you buy all your books from Amazon, sure.