The world and idea of "A Darker Shade of Magic" is easily done just fine. There are four parallel London, where magic and people are quite different. One of them, Black London, perished at its overhang of magic. Grey London has almost lost all its magic, White London talks to power struggles alive, only Red London seems to flourish. Kell is almost the only link between the countries, because "Antari" are very rare. They are the only ones who can create goals by blood magic and change the worlds.
So far so good and everything also pretty awesome. Only Unfortunately it took me quite a while until I arrived in history. The first 100 pages contain a lot Wordlbuilding, lots of character introduction, but rather little action. Truly captivating it is only when meet Kell and the thief Lila. From there, the story picks up speed and was much better.
My problem was the book that my characters unfortunately seemed very wooden. Before my eyes they were never quite alive and were the story for me not rousing wear. Although I have understood the motivations of the characters, but for me it was applied more and planned across. For me, the characters have their emotions only words, not lived. That's why it was hard for me to inspire me really for the book and also towards the end it was unfortunately only in order and not outstanding.
But that's a very subjective sensation that at another reader can play completely different again. Because basically the book has everything you need for a fantastic story. For me, the spark did not skip without that I can just point a finger on why.
Conclusion: "A Darker Shade of Magic" is certainly a good book with a well-told story. Technically everything is OK, the seed is set and just waiting to be working. For me it did not work, the characters are for me unfortunately always distanced a bit and remained wooden. There are motives and motivations, but me they were not internalized by the characters, but more kept narrative the reader's nose. The lack of reference has made it difficult for me and let the book (for me), unfortunately, only a little above average returns.