This is a terrific fantasy novel, set in London in 1819. Except that there are 4 versions of London, completely different worlds so multiple parallel universes. A small number of magicians can travel between the different Londons, but there is black magic and tragedy. Full disclosure, there is a significant kill count with collateral damage to some very sympathetic characters (the kindly innkeeper, for example), so this is not Harry Potter magic. Best of all, there is a great character called Lila, a feisty pick-pocket and wannabe pirate. This is a very imaginative and enjoyable read that is often philosophical; thanks Amy for this recommendation.