A Darker Shade of Magic
Shades of Magic - Book #1
by V.E. Schwab
I'd heard a lot about V.E. Schwab's Shades of Magic series, and when one of the recent books in the series was nominated for a Hugo (sci-fi award), I decided I should pick it up and start it. It's really creative, and the two main characters are really compelling. TL;DR I liked it.
In this universe, there are multiple worlds. Think of them as stacks of paper, one on top of the other. In each world there is a London, while almost everything else changes.
The protagonist, Kell, refers to the top London as "Grey London." Grey London has almost no magic -- essentially, it's our London. The bottom London is "Black London," and it's almost (if not completely) pure magic. The history behind this is fairly mysterious at this point, but it has been locked away from the rest of the worlds to ensure the other worlds' safety. The other two worlds are Red London (practical, controlled magic) and White London (uncontrolled, primal magic).
I don't want to give too much away, but suffice it to say that Kell and one other person can easily travel between the worlds, while all other travel between worlds is forbidden. Then Kell inadvertently smuggles an item across the world barriers... and all hell breaks loose.
It's good. Pick it up.