While there are many books entitled City of Glass, it is the one by Cassandra Clare that is the one mentioned here (according to the topic of your question). This book is a Shadowhunters novel which is mainly about Clary, who is trying to save the life of her mother.
The "City of Glass" in the title describes where Clary must go to save her mom. It is the home of the Shadowhunters who are constantly worried about demon invasions. Even worse for Clary, it is against the law for her to go to the City of Glass. It isn't long before Clary finds that her friends Jace and Simon are either in prison or woking against her. Moving on her own, Clary begins to unearth ideas from her family's past with the help of a rogue Shadowhunter: Sebastian. Just as the Shadowhunters are on the verge of annihilation, we learn that both the Shadowhunters and the Downworlders must work together in order to survive. Jace and Clary get closer and closer as the novel progresses and do, in fact, save the City of Glass and Clary's mom's life.
In conclusion, it is important to note the amount of suspense found in this novel. Suspense seems to be Clare's strong point in writing. However, what seems to strike readers the most is the relationship (often strained) between Jace and Clary.