I keep changing my mind between 2 and 3 stars.The underlying idea is so fascinating but the execution so poor! Four distinct London, existing contemporary in time and space but in four different dimensions, separated in fate by the presence or absence of magic and the use made of it. Our protagonist, Kell, is one of the last two Antari, the only people who can travel through the worlds for... well... sending correspondence? 
 To be honest, at the beginning of the book Kell seems little more than a gifted postman and it's not clear why keeping contact between worlds sealed one to the other. 
 Magic itself has very poorly defined contours, who can do what and how is not clear and the difference between and Antari and any other person who can use magic can't be really appreciated. 
 In the beginning Kell seems a fascinating character, mysterious and cunning but, as the story unravelr, he becomes more ordinary, the well-meaning but a bit naive hero that we usually find as a protagonist in this kind of story. His counterpart, Lila, is even more stereotyped: our usual tomboy with a sad past,you can tell from page one that she will become Kell's troublesome sidekick. Schwab tries to add some banter between the two but the result is quite dull. 
 Another character not quite convincing is Kell's brother, Rhy, whose presence seems justified only to satisfy the need of a bisexual, irresponsible character, responsible for all the comic interludes f the story. 
 This said, Schwab manages to write a gripping novel, full of interesting prompts, that you'll struggle to put down. However too many crucial points are resolved in a simplistic and predictable way (like the part where Kell fights his internal battle against the black magic that threatens to envelop his heart and is saved by Lila's voice), as if the author couldn't decide between writing a story for adults or for middle graders.