This is book 2 of the Mortal Instruments series. In book 1 Clary learned the identity of her father as well as the existence of the arcane world around her. Her father was able to steal the first of the Mortal Instruments, The Mortal Cup.
In book 2, Clary's father, Valentine, is after The Mortal Sword. Clary, Jace and their allies learn why Valentine is collecting the Mortal Instruments and they do their best to stop him. At each new twist and turn, Clary and Jace learn more about their unusual abilities and why they have them.
This is still a pretty good series. Clary is getting a little tedious with her teenage whining over her love for her apparent brother Jace and her love for her best friend Simon. Since this is young adult fiction, I guess I can't complain about that too much. There are a few plot twists that are set up really clearly that it's annoying that the characters don't see it - again, young adult fiction.