I get this question all the time, I think because a lot of people know that Tessa appears briefly in City of Glass. (She is the girl in white Magnus is talking to at the party in the epilogue.) But unfortunately to answer the question any further would involve too many spoilers!
The Ragpicker King is the sequel to Sword Catcher. In it, Kel — now fully embedded with the Ragpicker King and his crew — tries to solve a mystery: Who’s responsible for the murders in the Shining Gallery? If someone’s plotting against Castellane, who is it? And why is the woman he’s loved since he was a child, Antonetta Alleyne, determined to marry a monster?