Kindle highlights may be used by Amazon.
If you're anything like me, at some point you started to run out of room for your dead-tree editions and started downloading e-books. While you no longer have the tactile experience of reading e-books you have to admit that having a fixed-sized device with which you can store hundreds upon thousands of texts makes life a lot easier, plus, not everyone can read comfortably on a laptop or desktop display. Enter Amazon's Kindle, the darling of the e-book reader market which not only lets you buy e-books wirelessly (which can either tank your bank account or save your sanity while stuck in traffic) but also lets you highlight passages and attach notes to them. It takes a little getting used to this functionality, especially if you're like me and annotate most every book you own to some extent with markers and pens. It should be mentioned, however, that a recent software update for the Kindle pushed out by Amazon causes your stored highlights to be sent back to Amazon for use by their "Popular Highlights" function.
This means that if you then browse the entry for an e-book you've been marking up on your Kindle, you might just see something you denoted show up. You might not be able to identify it as yours per se but if enough people flag the same text chances are that will push it higher up the list. I just gave this a try on my Kindle by turning the wireless function on (I keep it off to conserve battery power), and while it's possible to browse your annotations by clicking on the "My Clippings" entry in your list of books (which is pretty cool, actually) thus far I haven't seen anything show up on Amazon.com, nor have I found a setting for opting out of this feature or even for backing up one's annotations. I haven't even found the "Popular Highlights" field of any book entries. While this doesn't mean that it's not there it is something worth keeping in mind if you're concerned about your privacy. You should also remember that the notes you take and the passages you mark off might be of interest to anyone who shows up on Amazon's doorstep with a handful of paper bearing a judge's signature because, after all, practically anything can be used against you with sufficient creative thought, and that those subpoenas can be issued just because someone got it in their head to go looking, regardless of whether or not anything is actually going on.