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. [loc. 66]
There are several Londons, in different worlds: the one we might think of as 'ours' is Grey London. Kell, an Antari blood-magician raised as a prince's foster-brother in Red London, is one of the few who has travelled to Grey London (where mad King George III reigns) and White London (the latter a starving post-apocalyptic wasteland) and knows the stories of Black London, destroyed by the magic it embraced. Kell is a courier between the rules of the different cities: he's also a collector and smuggler of the unique, from Grey London music-boxes to thaumaturgical texts.
On one of his expeditions to Grey London Kell meets Lila, a pickpocket who's convinced she was born to be a pirate. The two of them are thrown together when Kell is set up by an enemy and finds himself in possession of a dark artifact -- one which threatens the boundaries between the worlds, and the nature of reality itself.
I very much enjoyed V. E. Schwab's Vicious, a tale of superhero origins and friendship betrayed. I'm not as interested by this fantasy novel, though the characterisation is good and the world-building fascinating. It's a good read -- Schwab is good at dialogue and pacing, and her prose flows nicely -- but feels less innovative. That said, I will probably read the sequel, A Gathering of Shadows, even though -- thankfully -- there are no cliffhangers in this first volume baiting me to do so.