Who's telling this story? To be honest, we're not sure. It's definitely an outsider looking in, who can zoom into any character's thoughts at will. That makes the narrator a third person omniscient one. He's got VIP access to every character's thoughts, which means we do, too.
A third person omniscient narrator is the perfect choice for this fantasy because it adds to the magical feel and far-reaching scope of the book. It would be kind of a letdown if we learned about all these awesome magical powers pretty much every character in the book has, but then we're limited to seeing only what was going on with Sophie and Josh.
With this third person omniscient narrator, we can shift between the twins' vantage points (especially check out Chapter 1 and 2 where we flit from the bookshop to the Coffee Cup and back again), see what's going on in the Shadowrealm, check in with Perenelle in prison, and see what plans Dee's conjuring up for the future. The P.O.V. here is nothing short of magical.
P.S. Interestingly, the omniscient narrator also gives us access to Dee's thoughts and feelings. It's not every day that we get such an intimate view of a villain, and we have to say, we kind of like it. Sometimes we find ourselves identifying with, even understanding Dee, much to our horror. Could this be a hint of shifting motives and alliances to come?