Jace is now a servant of evil, bound for all eternity to Sebastian. Only a small band of Shadowhunters believe he can be saved. To do this they must defy the Clave. And they must act without Clary. For Clary is playing a dangerous game utterly alone. The price of losing is not just her own life, but Jace's soul. Clary is willing to do anything for Jace, but can she still trust him? Or is he truly lost? What price is too high to pay, even for love? (Walker Books)
Unlike in the 4th book, Jace is not really possessed but Sebastian can control him, sort of. Jace is more like a Jace-Sebastian-Hybrid who thinks Sebastian is awesome. They are bound by blood, so if you kill one of them the other one is going to die as well. Clary, Alec, Simon, Magnus (awesome as ever), Isabelle, Jordan, and Maia now have to find a way so separate them without the help of the Clave.
The book is not as good as the first ones but here I liked that the focus was not just on Clary and Jace but on the others as well, it wasn't so one-sided. One thing that bothered me, though, was that Clary was really dumb, and I mean really really dumb. And Alec too.(show spoiler)
But the rest was good and I'm curious for the last book in the series and how it all turns out. Some things are clear but not all of them. Let's wait and see.