X-ray was the feature that I was most anticipating and I'm just sorry that it is not currently available in more books. This morning as I'm 70% through a book, a minor character from an earlier chapter made an appearance and it was so cook to be able to use x-ray his name and refresh my memory.
When one goes to x-ray, the characters on the page appear in a list. Next to the list is a shaded horizontal bar with a vertical line bisecting it at the point current point of reading. There are shorter vertical bars at each instance that the character appears. When it's a major character, it just appears as a solid black block. By pressing on the unread area of the horizontal bar, it takes one to those references.
In my case, I was on page 219, so I didn't look at any of the references past that. I don't think it is much of a spoiler since one does have to make an effort to page down to see all future references.
The spoiler is in the graph itself, not just the references. I don't necessarily want to know that a character is a major character ahead of how it is revealed to me by the author, or particularly that he is a minor character now, drops out of the story completely until the end, and then plays a major part at the end. Things like that should just be allowed to unfold without you being aware of what is to come. (Of course, this matters only in fiction, and it would be quite a different matter in non-fiction.)
So that said, I found mention of this feature in a review
http://eptiger.blogspot.com/2011/11/kindle-touch.htmlwhich states:
"It also shows you a blueprint of where mentions of the character or term occur in the book and you can easily browse these selections. It's quite fast and quite accurate as far as figuring out what people and terms are relevant. You can show these lists for the current page, chapter, or whole book so it's easy to find what you're looking for. "
So perhaps in the future they could tweak this feature so you have the option of limiting what is reveal in that bar graphic (and search results) to just what you have read up to that point. That would be a great improvement for when reading fiction I think.