“Kell wore a very peculiar coat. It had neither one side, which would be conventional, nor two, which would be unexpected, but several, which was, of course, impossible. The first thing he did whenever he stepped out of one London and into another was take off the coat and turn it inside out once or twice (or even three times) until he found the side he needed. (A Darker Shade of Magic by V. E. Schwab)
Though many authorities have various official ways of dividing up the fantasy genre, my home-made system starts with classing all fantasy as either ‘background magic’ or ‘systemic magic’. Background magic books are really an adventure or mystery story dressed up in a fantasy costume. Their magic is simply a by-product of the atmosphere and new magical rules can be created whenever the plot needs them. This sub-genre has several wonderful examples, including Harry Potter, but their logic topples easily at the slightest thought. Systemic magic fantasy, on the other hand, has a specific magical system, with limitations and rules that are explained and followed, even when inconvenient (think Mistborn or Powder Mages).
Of the two subdivisions, A Darker Shade of Magic is most certainly the former. The magic makes almost no sense, simply borrowing from the general idea of elemental magic and throwing in a few extra worlds and some blood magic for effect. As a result of the hastily sketched magic, the fight scenes come off as uncertain, as if the characters have as little clue what their magic can do as the author does. Though a gratifying adventure/world-saving story, this book will not satisfy fantasy fans with a craving for new, creative magical systems.