City of Bones
Clary Fray witnesses a murder in a New York nightclub but she is the only one who can see the killers. Her mother disappears the same night. Clary is pulled into a hidden world of Shadowhunters, demons, warlocks, vampires, and faeries.
City of Ashes
Clary is deeper in the Shadow World as Valentine seeks the second of three Mortal Instruments. The central love triangle becomes more complicated. Faster-paced than Book 1 once it gets going.
City of Glass
Originally intended as the trilogy finale. Takes the main cast to Idris for a climactic confrontation with Valentine. Resolves the central conflict of the first three books. Works as a stopping point before the second trilogy.
City of Fallen Angels
The second trilogy begins. New threats emerge. Reading it is optional if you were satisfied with Book 3.
City of Lost Souls
Jace is missing and Clary must find a way to save him from Sebastian. Widely considered the weakest of the six.
City of Heavenly Fire
Sebastian's plan reaches its conclusion. Brings in characters from The Infernal Devices for a crossover that rewards readers of both series.