Clockwork Angel: Can someone give me a brief summary of what happens and the Characters?

Candra Clare is coming out with the 4th book in the mortal Instruments series ( City of Fallen Angels) YAY! She advised people to read Clockwork Angel because some Characters are in the 4th book that was in Clockwork Angel. Can someone please tell me about it! My library doesn't have the book and I already spent money for the first three books of the Mortal Instruments :) Thank You