This book had a lot of good things and incredible potential, but I think it was too ambitious for its own good.
The story follows Rosemary, who grew up with stories from her aunt about another world reachable by a pocket watch. The world has different segments, called hours, where the residents have different magical powers. After her dying aunt gifts Rosemary the watch, Rosemary enters the world and finds that it is under a dystopian oppression, which only she can save it from.
Rosemary can only enter this world when she is asleep, so the story is told with Rosemary flitting from the waking world to the fantasy world. This gives rise to an annoying mechanic where Rosemary is constantly going to sleep, waking up, going to sleep, waking up. It's used a lot as a way of building tension, ending a scene, or getting her out of trouble in the first half, though fortunately the second half uses the mechanic a bit more interestingly.
The world building is very cool. The landscape is oddly colorful, twisted, and strange. I love how magic is represented by smoke and how each realm has such a different look and flavor to it. The story and some events that happen are a little darker than your typical middle grade story, but I don't think it's especially so. Unfortunately, the story moves very quickly and by the end much of the world feels very underdeveloped.
A few more minor things:
The story waits until the end to say the age of the main character, and I spent a lot of the first half wondering if I was reading a 5th/6th grader or a 9th/10th grader.
One of the characters is referred to as 'they', which wouldn't have bother me except that they were described as being a 3rd/4th grader, so eight or nine years old.
There is a bully character who is pretty generic.
The story uses both the 'loner, bullied outcast' and 'chosen one' troupes & doesn't do anything new with them.
MINOR SPOILERS TO FOLLOW
The main downfall of the story is the pacing. At no point does the story move slowly, but it wasn't until the second half of the book that I realized the entire first half of the book was the set up and not the actual plot. After spending the first half introducing the new world and the stakes, Rosemary has to go and solve the problems, which involves her visiting five other realms (at this point we've visited one). So the story, which was already moving at a breakneck pace, goes even faster to complete the quest objectives. The result is that each of the new realms visited don't get any time to develop and the quest, instead of feeling grand and dangerous, feels trivial because of how quickly the objectives are accomplished.
By the end of the book, a lot of the first half felt irrelevant. The characters we met play little, if any, role from the 50% mark onward, and a lot of the conflicts, like Rosemary being bullied, her aunt running away, and her estranged relationship with her father, don't matter and aren't even fully resolved by the end. The second half then introduces new plot points and conflicts, such as a scandalous family history, and never fully explores, explains, or resolves them.
I think if less time had been spent on the first half of the book, and that had been the first third or quarter, or if the book itself wasn't as short, then the world could have been much better explored & the plot better fleshed out.