The Electronic Frontier Foundation can help.
They've just released the 2012 Edition of their eReader Privacy Guide, and it chronicles all the spying that is going on in the major US ebookstores, with the surprise addition this year of the Internet Archive and IndieBound, the ABA's own reading app.
Want to know who's tracking what other books you're reading on your ereader? Do you want to know who shares what with which third parties? (It's complicated) This chart can tell you.
Note: The chart is missing at least one of the major ebookstores, iBooks. It's not clear why the third largest ebookstore was omitted, but when you add that to unanswered question about the inclusion of Internet Archive (it's a website, not an ebookstore) and Adobe Content Server I do have to wonder at the technical knowledge displayed by the legal intern who did the research.
I've got the feeling that the EFF should have run this by an ebook geek before posting it. Now that it is up, I'm betting that it is going to go through at least one round of revisions for relevancy, and by the time I'm done reviewing it it will likely need to be revised for factual errors.