This was a fun read and a neat addition to the stranger things universe. A great way to give added content to fans of the show of most ages. The writing falls flat a few times which is unfortunate but that should be expected from a TV spin-off. People say there are plot holes but honestly I could only spot maybe one and it became a plot hole after future seasons were released so pretty reasonable in my books.
*spoilers*
My first big critique is that they spent way too much time character building Andrew with absolutely no payoff. Opposite to the duffer brothers, the tragic one off character of this 'season' gets nothing more than a 'you didn't hear?' From his parents. Big missed opportunity in my opinion.
My second critique was the 'plot hole' that Kali didn't know the other subjects (again not really revealed to be a plot hole until season 4), and that we don't get her escape shown in this novel. I would be shocked to see her return in season 5 and even more shocked for the Duffer Brothers to give her enough backstory to include this somewhat crucial info. Perhaps the way Kali escaped ties into Jane's escape for example.
Again all in all I thought this was a very enjoyable, light and easy read. I love that they made novels for this universe because to me the logo for the show looks like it belongs on a 70s mystery novel. I would give this a B- to a B rating.