My six-year-old has developed this very polite habit of calling things she doesn't like "not my thing." This was "not my thing." It's hard to say quite why: I like fantasy and strong female protagonists and good world building, but. Here's the good: the world building was amazing. I loved the idea of the four Londons, and it felt truly original -- each felt real and whole. Schwab really excels at invoking a feeling of a place. But. The protagonists fell flat. There was too much violence -- pretty much anyone named and not protected by plot armor died within the chapter, and when that happens it's hard to get attached. The plot felt flimsy.

Also, I know it's itself derivative to accuse a fantasy book of being a Lord of the Rings derivative, but the MacGuffin was an amazingly powerful malevolent artifact that enervated the users, but whose possession and use was addictive. Using the MacGuffin (primarily to be invisible) allowed the enemy to spy on them. The main quest was to return the MacGuffin to the place where it was made in order to destroy it.

Yeah, not my thing.