What are the Infernal Devices about? How are they related to the Mortal Instruments?
The Infernal Devices are a trilogy of prequels to the Mortal Instruments books, set almost 130 years ago. They deal with the adventures of a Downworlder girl named Tessa in a Victorian London where the Accords have only just been finalized and tensions between Shadowhunters and Downworlders are running high. It’s a romantic adventure centering around the Lightwood, Herondale, Wayland, and other Shadowhunter families you’ll know well from Instruments; Magnus Bane appears in them, and I generally recommend you read Clockwork Angel before City of Fallen Angels, as the characters from one series connect to the characters in the next (though, as it says here, neither series spoils the other.).