A Darker Shade of Magic

By V.E. Schwab

Four Londons: Black London, White London, Red London, Grey London, existing in separate universes with few similarities. Doors connect these Londons but only special people, antari, can access the doors by using magic.
Black London has been overrun by magic and to protect the other Londons, it was sealed off. White London is starving for magic and, as a consequence, its people are twisted and corrupt. Red London has bountiful magic and people are happy and prosperous. Grey London is ordinary and not magical, although magic is available to those with talent.
Kell is from Red London and he is an antari. He serves as a messenger, traveling between the Londons, enabling the rulers of each London to stay in contact.  But Kell, who is a bit resentful of his fate, has turned smuggler too, bringing trinkets from one London to another, although this is forbidden. It is his smuggling that brings him to grief in Grey London, when he falls into a trap and is nearly killed but is saved in the nick of time by Lila Bard.
Lila is a thief and a pickpocket. She stole a stone from Kell, an stone that the rulers of White London desperately want. It is a stone from Black London and it is saturated with powerful magic, magic so strong even Lila is able to conjure using it. When she stole it from Kell, he tracked her down and retrieved the stone. Holland, who is the antari for the rulers of White London, takes advantage of Kell's sympathetic nature and tortures Lila, expecting that Kell will come to her rescue, which he does. Holland then nearly kills Kell, trying to get him to give up the stone and Lila simply bonks Holland on the head and knocks him out and saves Kell from certain death.
Now it is up to Kell to figure out why Holland and his White London bosses are ready to commit murder to possess the stone. Together he and Lila will discover a vile and sneaky plot that threatens to destroy Red London.

This was a good read, if a bit convoluted at times. The only thing I found disappointing was that we never get to see Black London for ourselves, even though it is constantly referred to in the story. This book seems like the first in a new series, so perhaps the author is saving Black London for a later book.
I also think the book is aimed at young adult readers more than a general audience. Both the main characters are quite young, Lila is nineteen and Kell is in his early twenties. And there is no "mature content," just a couple of chaste kisses.