A Darker Shade of Magic is a surprisingly fantastic book–and I mean that in the true sense of the word fantastic.
The core worldbuilding conceit of the book is that of four parallel worlds, specifically four Londons. Gray London (our London?), a dull world with little to no magic left; Red London, with magic full of life; White London, with a more sharp and controlling feel; and lost Black London. The feel and description of each London really makes the book, with Schwab evoking each London with colors and scents and even feelings. It’s a really interesting take and done rather well.