I will tell you right now, I am not a huge fan of the fantasy genre, but I LOVE this book!
Kell is one of the few left who has the ability to travel between Londons. Yes, there are multiple Londons. There is Grey London, which we will recognize as the London we know and it also lacks magic. There is Red London, where Kell is from and magic is everywhere. There is White London, ruled by ruthless twins where everyone is willing to kill for any magic they can find. Finally, there is Black London which had been consumed by magic and sealed off for some time.
Kell happens to smuggle items between worlds. Despite his illegal activity, he avoids trouble until he ends up with an item that no one should have. In his attepmt to flee, he ends up running into Grey London's most wanted thief, Lila Bard who lives up to her reputation as she pickpockets Kell.
The two are dragged into the power and the danger of magic as Lila gets the adventure she has always longed for but of which she could never even dream.