Like Neverwhere by Neil Gaiman and Un Lun Dun by China Mieville, this story is set in a multiverse with more than one London. Unlike those stories, V.E. Schwab's has four Londons, not two. When the story opens, travelers between her Londons are extremely rare, because only magic users called Antari can cross over, of which, at present, there are only two: Kell from Red London and Holland from White London. They deliver messages between the rulers of Red, Grey and White Londons. The fourth London, Black London, is closed off, because magic is out of control there. (The city names are not official. They are Kell's. Read his explanation someday if you haven't already.) 
 The four Londons differ dramatically. In Grey London, magic is mostly forgotten, like in the England of Jonathan Strange & Mr Norrell by Susanna Clarke. A young thief in Grey London, Delilah Bard (Lila), tells a lot of the story. About a quarter of the way through, Lila meets Kell, who also tells a lot of the story. 
 Both narrators are interesting. Kell is a member of Red London's royal family. As an Antari, he delivers messages. He has a very bad habit though, which creates a problem involving a magical object, similar but different than the Ring from Tolkien. Unlike Frodo or Bilbo or Gandalf, Kell recognizes he has a huge problem almost right away. He accepts responsibility, but his background and disposition did not exactly prepare him for the task. Maybe you too will root for him while wanting to kill him. 
 Lila's problems are more commonplace or systematic. Poverty. No social mobility. (Violence and neglect.) It shapes her. She is lovely and cold-blooded. Whew, the chip on her shoulder is even bigger than the one I had at her age. She is perhaps the best, the most believably strong woman I've seen in fantasy - not that I've read exhaustively. 
 If you're into magic, the magic in this book is curious and scary, except when it's not. Red London is full of magic, but without the darker shade that exists elsewhere... 
 The first half of the story, at least, probably more, was 5 stars fabulous. V.E. Schwab combines all these elements into a great tale. Some of you will roll your eyes and curse my knavery, but the last third or so actually suffered, I think, from too much action! The character development I craved was forgotten, and I'm not talking about romance because there is none. I'm itching to say something about Lila's fate at the very end but it would be too much of a spoiler. And Kell's fate too actually. (view spoiler)[Kell's action brought more than mayhem to Red London. People died! He gets off scott free because he's royalty?! And Lila? She gets nothing. Walks off without a thank you from the King and Queen. Hmmph! (hide spoiler)] That abbreviated development or lacking elaboration, perhaps, is what made the story seem short to some people, including me. I'm not the type to start A Gathering of Shadows tomorrow, but I will get to it eventually, because of Kell and Lila.