Cassandra Clare is the author of City of Bones, City of Ashes, and most recently, City of Glass (McElderry), the final installment in her Mortal Instruments trilogy. Clare spoke with Bookshelf about playing character favorites, making promises to fans, the ups and downs of saying goodbye to a big story, and working on a new series.

Are you sad to be finished with this trilogy?

I am sad—it’s sort of a postpartum feeling. You work so hard for so long and are so involved in the world you’ve created that when it is all over, at first you think, This is so great! I’m done! I never have to look at this again! Then, once it’s all bundled off, you get all sad and nostalgic because you realize that it’s really over. My editor feels the same way. I’m mostly about saying goodbye to the characters—I got pretty attached to them.

Speaking of getting attached—do you have a favorite character?

I’m not really allowed to have favorites—it’s like having favorite kids. Though I’m very attached to Jace and Clary and their story is really over with City of Glass. I wanted to make absolutely sure to end their storyline with this book—to give resolution to Jace and Clary, good or bad. So I will miss Clary and Jace. And Simon, too. Simon is my favorite—if I’m allowed to have favorites.

Why Simon?

Well, Simon is the character that’s most like me. And he is also the character that keeps you grounded in reality—he is a human being whereas Clary is not. Even though Clary grew up as a normal human, she really belongs in the world of the supernatural because ultimately she is a supernatural creature—a Shadowhunter. Simon, on the other hand, is dragged into the supernatural world because of his ties to Clary, but he is utterly human. And it’s dangerous for him! Simon exemplifies why the Shadowhunters keep their world a secret—once ordinary people are involved, it puts them in danger. In Simon’s case, his involvement eventually kills him in City of Bones. Simon had to become a vampire to continue in the world. I was sad, though, turning Simon into a vampire.

What—if any—hints do you want to give fans of the series about this last installment?

I’ve always gotten a lot of questions about what is going to happen next, and I’ve always responded that people will find out everything in the third book. Whether readers like these storyline endings or not, they are going to get all the answers—I did not leave a lot of loose ends dangling. Most important of all, Clary, Jace and Simon kick some butt and really flex their muscles. Fans really wanted to see this happen and it does.

Was it intimidating to tie everything up?

Yes. The more questions you pose, the more work you have to do to finish things up. Even at the beginning as I was setting up the storylines, I thought to myself, One day I will have to finish all of this. People warned me that the last book would be the most difficult because you are stringing readers along with a lot of promises. But then, when you actually get to answer the promises, it’s very satisfying.

What’s next for you?

I’m working on a new series and it’s also about Shadowhunters. For now, it’s called the Infernal Devices—I don’t know if the title will stick, though, and I’m bad at committing to titles. It’s actually a prequel trilogy in the sense that it deals with the ancestors of the kids in the Mortal Instruments series, going back about 150 years. There are a few characters that overlap between the trilogies, like Magnus the warlock, because he’s so old, and the Fairy Queen, also for the same reason. But it features a new group of kids—it centers on a girl named Tessa who is a warlock. She meets a group of Shadowhunters at one of the Institutes in the year 1878, and it switches back and forth between London and New York.

So technically I didn’t have to give up the world and the magic system I created in Mortal Instruments, and I’m really enjoying working on it. My first draft is due in May. It’s set to come out in fall 2010.

Any chance of a fourth book in the Mortal Instruments series?

If I did another book in the Mortal Instruments series it would be about Simon. By the end of the third book, he’s essentially a superhero. At one point I did an outline of a book that involved Simon going off to do a music school program in a small town with the rest of the characters slowly following along. Yeah—if I did a spinoff series it would have to be about Simon.

City of Glass by Cassandra Clare . McElderry, $17.99 ISBN 978-1-416-91430-3