The X-Ray feature on Kindle is really handy for epic books with a large cast of characters or those stories that span years, where a character can disappear for pages and pages and then pop back in again. X-Ray give you a idea how important the character is, and who they are, but it only works if the publisher has gone the extra step and provided Amazon with a file to use to generate the X-Ray.
In addition to the newly-purchased Goodreads, Amazon also owns social reader site Shelfari, and one of the things this means is that on Shelfari, the author or any reader can create lists of characters and places in the book. Those lists are then available to view on the bottom of the product page on Amazon.
It looks like Amazon might be planning to spin this function off into creating X-Ray files for KDP books, because my book just got a whole bunch of characters added by someone known as "X-Ray," who has also added characters to a boatload of books!
It would be cool if it's true!
Update: I just downloaded this book again and it now has the X-Ray feature! Yay!