A Darker Shade of Magic, by V.E. Schwab

 A Darker Shade of Magic is a rollicking fantasy yarn. Not too deep, but pleasantly put together. Four parallel earths exist, and special magicians can travel between them. Each has a version of London, but otherwise they are quite different. Grey London is fairly indistinguishable from our own -- magic is largely dead. One of the magicians is tricked into receiving a Macguffin from Black London (which has been eaten up by magic), but before he can return it (somehow) to where it belongs and can't hurt anyone, it is lifted by a lightfingered guttersnipe wannabe pirate lass. Naturally, there are other interested parties, and a violent magical chase ensues through various Londons. Good summer fun, but I doubt I'll continue the series, despite the blandishments of the publisher.