It is the series that comes between TID and TMI, that takes place around 1903. It features the children of the characters from The Infernal Devices: Jem, Tessa, Will, the Lightwood brothers, Sophie, Cecily, etc. You can read more about it and see pictures of the characters here.
“Full measures indeed of love and suffering…and bloodshed too: Fans will sail through and clamor for more.” — Kirkus