A Gathering of Shadows is the sequel to Schwab's A Darker Shade of Magic; it picks up four months or so after the end of the first book, when Kell bound Rhy's life to his to save it and Lila danced off into the distance of Red London. Now, the thrice-yearly Element Games are approaching, a Triwizard-ish tournament in which thirty-six magicians from the three great empires of Red London's world compete for honour, glory and a large amount of cash. It's a significant political event as well as a sporting one, which means that Red London's royal family (including Kell and Rhy) is under unusual pressure to perform their parts.
And Lila is returning to London with her flamboyant privateer captain Alucard. Needless to say, Lila, Kell and Rhy get embroiled in the Games pretty quickly.
A Gathering of Shadows is deeper, I think, than its predecessor: whereas Darker Shade concentrated mainly on Lila and Kell, Gathering hops through the heads of a number of characters as they collide in various ways, although Schwab is always careful to introduce new characters properly to us before we see things from their viewpoint. This wider focus is one of the great strengths of the book, I think: Schwab releases information to us in bits and pieces, at exactly the right time to keep the tension up. Which is good, because this also gives the book the illusion of pacing. As in Darker Shade, not much actually happens here, and the book's quite happy to wander down random tangents; but it's always in the service of character, and so it serves to enrich the world of the novel without putting in pace-killing infodumps.
The characters, of course, are the book's other great strength: Kell and Lila especially escape the typical fantasy labels of "good" and "evil" and manage just to be "people", who sometimes make good decisions and sometimes questionable ones, and make them for all kinds of reasons, selfish and unselfish. I desperately wanted more of Lila, the cross-dressing piratical woman from Grey London; not that we didn't get plenty of her, but she is so fantastic that I did sort of want the whole book narrated by her. Alucard, too, a new character for this book, is larger than life and fascinating.
This is very much a Middle Book, setting up plot for the series finale; but it never felt that way when I was reading it, and I'd be quite happy to stay in the world for twice the time I got in it.