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.

But barely a glimpse of them have been seen in the last seven years―and a new Antari named Kosika has appeared in White London, taking the throne in Holland's absence. The young queen is willing to feed her city with blood, including her own―but her growing religious fervor has the potential to drown them instead.

And back in Red London, King Rhy Maresh is threatened by a rising rebellion, one determined to correct the balance of power by razing the throne entirely.

Amidst this tapestry of old friends and new enemies, a girl with an unusual magical ability comes into possession of a device that could change the fate of all four worlds.

Her name is Tes, and she's the only one who can bring them together―or unravel it all.