Cinder, Scarlet, and Cress. And while we wait for Winter to be released this, well, winter... (I just preordered), this full prequel story was released. When finishing (and freaking out that I wasn't done with the story) Cress, I learned that there'd be a prequel release before Winter, but I research enough to who it was going to be about.
This was the prequel about the story's antagonist. A story explaining how we got to where Cinder began. I enjoyed learning a little more of the background of our characters. Just a few months to go to learn how Meyer finishes this fun tale.