This book has a sophistication rare either for "teen fiction", or fantasy fiction, in which genres it consciously lies. That sophistication means that it is likely to be enjoyed also by an adult audience, as well as its target young adult audience. It is written very much at the same level as Philip Pullman's "His Dark Materials" trilogy, ie, not suitable for such young readers as Harry Potter. And I believe that it stands up well to a comparision with the Pullman, which I recently read.
Dickinson has put considerable effort into researching the mediaeval "gothic" period, to create a realistic world of daily life, petty princes, warfare and monasteries, into which to place his story of magic. Writers, like Tolkien, who have such a coherent "back-story" are rare.
There is a realistic humanity to the characters. There is not the cannot-do-wrong hero or always-evil-baddie, (with the exception of some malign magical powers). Rather, whilst we have some clear heroes and enemies, all display only typical levels of self-interest, weakness, duplicity and generosity that any human will have. This is another level of sophistication rarely found in this genre.
Finally, the plot itself is sophisticated. There is no obvious best action that the hero has to choose. Rather, complexities are revealed to every action, as in the real world. Everyone desires unity to avoid war, but no one is willing to give up the power that they have.
On top of that is a page-turning plot that draws you in. Whilst the book stands on its own, the book ends with a tense situation that draws you in to the sequel.
Declaration of interest: the author is a friend of mine.