The story starts with Katie Lin who, while appearing human, is a bit more. Unlike the "Ever-Dying" ("humans" or as J.K. Rowling called them, "muggles"), Katie is magical. In her world, the "Ever-Dying" read about magic but don't think it's real, like many people believe about angels and demons in our world.
The book immediately kicks off with Katie on the run with someone from one of the other magical races without a clue as to why she's running or why she's being pursued. The rest of the book cleverly reveals what's happening in the background.
There was quite a bit of world building including multiple races and worlds in an urban fantasy setting and pocket universes if you still need more. The relationship between the races and their worlds and between the races, while derived from classic myth (with some mutation), was consistent and relatively balanced.
In fact, the book had to flip back and forth from the story to explanation about the races and worlds to provide the context for what was happening. This would be a great introductory book to the series where later books could focus just on the story.
The plot, characters, and settings were all plausible and well developed. Each character had its own voice. Many of the races drawn from classic mythology (e.g., the Seelie Fey) reminded me a lot of Kevin Hearne's books. Some of the faces (e.g., the Fey) might be contrasted with elves but in this world are quite different; "Fey are not woodsy. They're deep into tech; they love it, and they're really good at it." These differences added to the story rather than detracting.
Merlin ("Myrddhin") turns out to be Katie's uncle. There are dragons, centaurs, a naga (and other creatures) in the book; there are even "merlions". At one point, Katie kept her belongings in a duffle-bag of holding. "The Hush" sounded like something out of a recent season of Doctor Who.
I enjoyed the writing in the book. "...the unexpected only works once." "Response is indicative of essence." "...exsanguinates the grandeur." +1 for using the word "kajigger".
One of the phrases used in reference to the book, referring to Katie Lin, is "...and one seriously underestimated damsel". This needs to be on the cover.