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.

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.

Elspeth Spindle needs more than luck to stay safe in the eerie, mist-locked kingdom of Blunder—she needs a monster. She calls him the Nightmare, an ancient, mercurial spirit trapped in her head. He protects her. He keeps her secrets. But nothing comes for free, especially magic. When Elspeth meets a mysterious highwayman on the forest road, her life takes a drastic turn. Thrust into a world of shadow and deception, she joins a dangerous quest to cure Blunder from the dark magic infecting it.

Ever since Cass almost drowned (okay, she did drown, but she doesn't like to think about it), she can pull back the Veil that separates the living from the dead...and enter the world of spirits. Her best friend is even a ghost. So things are already pretty strange. But they're about to get much stranger. When Cass's parents start hosting a TV show about the world's most haunted places, the family heads off to Edinburgh, Scotland. Here, graveyards, castles, and secret passageways teem with restless phantoms. And then Cass meets a girl who shares her "gift"....