A Gathering of Shadows is the sequel A Darker Shade of Magic written by V.E. Schwab. It also appears to be the second in a series of three books1, because three is a magical number (also refer to recent popular book and movie success The Hunger Games, and somewhat less successful Divergent series).
It picks up from the end of the last book, four months after Kell and Lila parted ways in Red London. Kell has no idea where Lila is or what has happened to her; he just assumes she has gone off to board a ship to explore more of the world as she had told him she wanted to do. Meanwhile, he is slowly unravelling being cooped up inside the castle, his movements limited and monitored by the king due to the events in the previous book.
And now here I must cut off the rest of the post with a warning: there will be a few spoilers underneath, at least regarding character development. I will still try to keep most of the plot unmentioned.