WARNING: SPOILERS FOR THE FIRST THREE BOOKS AHEAD.
I had indeed initially planned not to write more Mortal Instruments books after City of Glass. Two things happened to change that: One, I had written a plot for a graphic novel about what would happen to Simon after the events of Glass, which is why I left so many threads untied at the end of City of Glass (where was Sebastian’s body, what would happen with Simon’s love life, the Seelie Queen’s threat, etc). When the graphic novel didn’t work out, I was left with this storyline and nothing to do with it — it wasn’t enough for a whole book on its own. However, while I was writing the first book in The Infernal Devices, Clockwork Angel, which deals with Jace, Clary, and the Lightwoods’ ancestors, the way events played out in it gave me the idea for a new villain and conflict that might beset the cast of characters from The Mortal Instruments, and connect up to the plotline from the planned graphic novel. I’ve always liked stories where the distant past comes forward to affect the future, so, without being spoilery, when I realized I could connect the events of Infernal Devices to the few loose ends left at the end of Glass, I realized I wouldn’t want to pass up writing that story, especially considering how much chaos I knew it would bring
to the lives of Jace, Clary, Simon, Alec, Magnus, Isabelle and the rest!
Then, in October of last year, I sat down to start writing the story of City of Fallen Angels. I had a detailed outline based in part on the graphic novel idea I had had, but when it came to expanding the outline and writing the story, it just wasn’t working for me. I was on a writing retreat in Mexico with a number of other writers, and when we sat down to go over the issues I was having, I realized that the story I had thought I was telling was really a much bigger story — that my smaller, Simon-centric story had morphed into something much bigger, much more epic, and deeply involving the whole cast of characters from the first three Mortal Instruments books. I realized that what I had on my hands was not a single book that would wrap up the story begun in The Mortal Instruments, but rather the beginning of a new trilogy about these characters. (The fun part was calling my agent and editor to explain “You know that one book I was going to write? Well, actually, it’s three books!” I like to think I could hear heads hitting desks all through Simon and Schuster. But when I submitted the outlines for the new Fallen Angels, City of Lost Souls, and City of Heavenly Fire, they were thrilled with the idea of the new trilogy — and I hoped readers would be as well!