City of Ashes is the second installment of The Mortal Instruments series for young adults. The book is very long and tedious.
The book continues right after the events of the first book. Clary is staying at Luke's house while her mother is still in a coma. Jace has been thrown in prison by the Inquisitor, who works for the Shadowhunter government called The Clave, for suspicions of working with Valentine. Clary, Isabelle and Alec go to the Silent City to free Jace where they discover that the powerful Silent Brothers are dead and the Mortal Sword has been stolen.
Jace tells his friends that Valentine stole the sword. They visit the Seelie Court and make an arrangement with the fairies to help defeat Valentine. The Queen of the Seelie Court humiliates Jace, Clary and Simon by forcing Clary to kiss the one she desires to be freed from a spell, and it turns out to be her brother Jace.
Simon gets killed after a desperate visit to Hotel Dumont to reverse the side effects from the vampire blood he ingested as a rat in the first book. Raphael, Clary, and Jace watch as Simon transforms into a vampire. Meanwhile Valentine has been murdering Downworlders to complete a ritual with the sword that would enable him to control the entire demon population.
Valentine eventually captures Simon to use as a vampire sacrifice in the ritual. Clary and Jace go to the ship where Valentine is hiding out and a great battle between Shadowhunters and Demons breaks out. There are several casualties, but all the main characters survive. Clary learns that she has special strength and ability with her runes and destroys Valentine's entire ship. Simon turns into a daytime vampire after Jace lets him drink his blood to revive him. While Clary is wrestling with her feelings for Jace and recovering from the showdown with her father, she meets a mystery woman who tells her that she knows how to wake her mother from the coma.
The story ends with questions such as what will happen now that Valentine has control of the demon world? When will Clary's mother wake up? and what will happen to Clary's relationships with Simon and Jace?