I have a lot of mixed feelings about Unlocked, though most of them are due to the marketing. When book 8.5 was announced, I had a lot of theories for why it might be different. Were we finally going to discover what happened during the Great Gulon Incident? Did the dual POVs mean Sophie might die? I was excited to figure out what secrets might be hidden within the "bonus exclusive content" that supposedly we couldn't find anywhere else.
What this book was marketed as: fun secrets and stories that would be relevant to the plot and that would enhance the series, along with an actual installation to the series in the form of a novella
What this book is actually is: a summary of the first 8 books through the guise of character profiles and worldbuilding infodumps
As much as I like the fanart, the "extra content" that readers were promised is not "extra" to someone who has read the series from start to finish at least 4 times. I could probably recite the majority of the details from the first 600 pages of this book (and my sister can attest to this; my cousin and I explained the plot of books 3-5 to her to get her to fall asleep).
I wouldn't have an issue with that if this book wasn't necessary to complete the series. The novella that makes up about 1/3 of the book is required for an understanding of the next part of the series, meaning you NEED to read this one.
Regardless, I'm still not giving an official rating on this book. The bonus content section has provided a few fun experiences (making cinnacreme, feverishly researching the middle names of several characters, creating new theories, etc.), but I just wish the book was optional.

Here are the notes I took on Unlocked, and they may contain spoilers for this book and others in the series:
- (inside cover) Cassius's beach house is in Australia, and I want to see him do an Australian accent
- (inside cover) Canonically speaking, Everglen is right next to Florida, meaning that during the 1960s, it could've been nuked during the Cuban Missile Crisis, and yeah...that's fun to think about
- I read through Sophie's medical records (as written by Elwin), and there's no real evidence of him possibly being her father, but he may have hidden it well (he does note that he thought it was a prank when Alden first called him to Everglen to meet Sophie)
- (157/166) Wylie and Stina have spy balls... as does Grady, but I didn't write down the page number
- In the portrait of Elwin and Livvy...Livvy doesn't have a registry pendant, and I guess that kinda makes sense, but I would've thought she would've had a fake one at least
- Brant's name is redacted, and I've seen a lot of theories assuming that he's related to Kenric due to their middle names being so close
- (526) If Kenric washed the Washers, who does that job now? Emery?
- (702) Things in the Neverseen storehouse: drugs, spare clothing, scrolls, a book, fancy marbles
- (711) Rex wants to be just like Dex when he grows up :(