Let me start by saying, I didn't read The Master Undone. I didn't want to spoil things for myself because I've only read the first in this series. So, this is strictly Rebecca's Lost Journals.
This is a great companion for the series. As far as I can tell it gives you little bits and pieces that you don't get in the novels (at least the first one). You get inside Rebecca's head a little bit more and see where she's at.
It still keeps the mystery up of who "he" is. Although, at one point you think you know and then you think you're wrong again. So, you're still really not sure.
I will admit that I did kind of stall on reading the rest of the series. This has perked my interest back up and I keep thinking back to how the first one ended and how much I really wanted to read the next. I really pulled myself back from this series and I'm not sure why, but I think I'm going to be jumping back into it in the very near future.
*Copy received in exchange for an honest review.