(I really love the cover)
The books are set in London, but in a universe where exist four dimensions, so there are four London's: the grey, the white, the red and the black. The black London is almost a myth, there's no connection with them.
The main character is Kell, who is an Antari, this means that he can travel between the dimensions. He is importante because not everyone can do this. Kell is from red London, where he works for the royal family being a messenger for them, and also smuggling objects to other dimensions. On a trip to grey London, he mets Lila Bard. She dreams with a good adventure, so she forces to take her with him.
I like this saga because the characters are very human, they can get corrupted by the power of the magic. They aren't perfect heroes. Also I like it because the idea of a world with magic is awesome.