Okay I much prefer this universe when Ben is the narrator as opposed to Janelle! This novella covers some of the events in Unravelling (from Ben's perspective) and a little after, as a bridge to the second book Unbreakable.
In Undone, we get to see some of the events through Ben's eyes and I really liked getting to hear his take on things as I had some problems with Janelle as the point-of-view character. You see a bit more of why Ben feels the way he does about Janelle and it did make me like her a little better, I admit. Undone covers some events prior to the events in Unravelling which gave a little bit more back-story and then some of the scenes from the first book are given a new focus. It then goes on to what happens immediately after the cliffhanger we were left with at the end of Unravelling. I think there might be some overlap with Unbreakable too but will need to wait until that book is out to know for sure.
Without giving too much away, I am actually really glad that things didn't work out too great for Ben and Eli (Elijah) after the events of Unravelling. It opens up a whole new world of possibilities for Unbreakable and it has only made me more anxious to read it. I liked seeing more of Eli (I love that he is so snarky and I did like the fact that he wasn't too fussed for Janelle in the first book - was nice to have someone who wasn't singing her praises).
I kind of wish I had waited until after reading Unbreakable to read this as it may have worked better within the narrative, but I would definitely recommend picking this up as it does give new insight on the plot and characters.