This was a great backstory for the rest of the Dorothy Must Die series.
This was more of a direct sequel than the original novel in this series. It handles Dorothy, and how she gets from what we see at the end of the original source material to what we see at the beginning of this series.
Watching her go through her transformation was fascinating. It was much easier to hate her outright before reading this (admittedly, she is still not someone I really like), and I felt like it did a good idea handling the changes in her.
The plot was relatively light, but this was absolutely a character novella--the idea was "how did this happen" more than anything else, so handling her getting into Oz again and how she reacts to everything going on around her was, in actuality, the plot.
It's a fun, quick read if you like the series already. If you don't, definitely pick up the first book and take a look if the idea of an Oz gone wrong sounds interesting. I'll be reading the rest of the novellas for sure and I can't wait for the third book in the series to come out!