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.

Kell is one of the last Travelers - magicians with a rare, coveted ability to travel between parallel universes. As such, he can choose where he lands. There's Grey London, dirty and boring, without any magic, ruled by a mad King George. Then there's Red London, where life and magic are revered, and the Maresh Dynasty presides over a flourishing empire. There's White London, ruled by whoever has murdered their way to the throne. And once upon a time, there was Black London...but no one speaks of that now.

Kate Harker and August Flynn are the heirs to a divided city - a city where the violence has begun to breed actual monsters. All Kate wants is to be as ruthless as her father, who lets the monsters roam free and makes the humans pay for his protection. All August wants is to be human, as good-hearted as his own father, to play a bigger role in protecting the innocent - but he's one of the monsters. One who can steal a soul with a simple strain of music.

Yeine Darr is an outcast from the barbarian North. But when her mother dies under mysterious circumstances, she is summoned to the majestic city of Sky. There, to her shock, Yeine is named an heiress to the king. But the throne of the Hundred Thousand Kingdoms is not easily won, and Yeine is thrust into a vicious power struggle with cousins she never knew she had. As she fights for her life, she draws ever closer to the secrets of her mother's death and her family's bloody history.

Thor, the Norse god of thunder, is worse than a blowhard and a bully - he’s ruined countless lives and killed scores of innocents. After centuries, Viking vampire Leif Helgarson is ready to get his vengeance, and he’s asked his friend Atticus O’Sullivan, the last of the Druids, to help take down this Norse nightmare.