The Fragile Threads of Power by V.E. Schwab

Once, there were four worlds, nestled like pages in a book, each pulsing with fantastical power, and connected by a single city: London. Until the magic grew too fast, and forced the worlds to seal the doors between them in a desperate gamble to protect their own. The few magicians who could still open the doors grew more rare as time passed and now, only three Antari are known in recent memory―Kell Maresh of Red London, Delilah Bard of Grey London, and Holland Vosijk, of White London… Read more.