City of Fallen Angels will be the fourth book in the Mortal Instruments series by Cassandra Clare. The series was meant to end with City of Glass, but it was announced that in March 2010, a fourth book would be added. Amazon states that it will release the book on April 5, 2011.[1] The fourth installment will equally follow the stories of Clary Fray, Jace Wayland, and Simon Lewis,[2] despite rumors that the novel would only feature Simon as the main character. When asked about the change in point of view for the next novel Clare replied, “Simon has always been one of my favorite characters to write about, and I’m excited to get the chance to tell his story while revisiting some of the other characters that I love from The Mortal Instruments series. The enthusiasm for telling Simon’s story that I’ve received from the team at McElderry Books and Simon & Schuster has been great, and I’m excited to be working with them to bring a fourth book to the series.”
Clare describes the next book by saying, "City of Fallen Angels takes place after the events of City of Glass. It splits its focus between Jace and Clary and Clary’s best friend, Simon, and how he adjusts to life as a vampire, but it’s still an ensemble-cast story, and all the characters from the Mortal Instruments series appear in it: Jace, Isabelle, Alec, Magnus, Luke, Jocelyn, Maia, and many more. (It even helps to have read Clockwork Angel before you read City of Fallen Angels, because some of the characters from The Infernal Devices series do show up in it. However, it isn’t required.) In City of Fallen Angels, someone has been killing the Shadowhunters who used to be in Valentine’s Circle and leaving their bodies around New York City in a manner designed to provoke hostility between Downworlders and Shadowhunters. Internecine warfare among vampires is ripping the Downworld community apart, and only Simon — the Daylighter who everyone wants on their side — can decide the outcome; too bad he wants nothing to do with Downworld politics. Meanwhile, Jace and Clary investigate a mystery that has deeply personal consequences for them — consequences that may strengthen their relationship, or rip it apart forever. Love, blood, betrayal and revenge: the stakes are higher than ever in City of Fallen Angels."[3]
It was later announced that there will be two more installments now following City of Fallen Angels, titled 'City of Lost Souls', and 'City of Heavenly Fire'.