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 fromInstruments; Magnus Bane appears in them, etc.
You can read TMI first, or The Infernal Devices first. It doesn’t matter. In an ideal world, I generally recommend you read Clockwork Angel and the others between City of Glass and City of Fallen Angels, as it flows with the way the characters from one series connect to the characters in the next, but neither series spoils the other and it genuinely doesn’t matter which you read first.
There are three books in the Devices series. The first is Clockwork Angel, the second,Clockwork Prince, the third, Clockwork Princess. They have all been released and the series is complete.