Note: My answers contain a few spoilers, so please don't read this unless you've read the book!
Why do you call this a “companion book” instead of a “sequel”?
I’m trying to be as accurate as possible, so no one gets upset with me! I think people usually expect a sequel to have the same main character as the original book (though I can think of plenty of great sequels where that’s not the case). Ella isn’t the main character of Palace of Mirrors, and she doesn’t even show up until page 167. So I wouldn’t want anyone to read the first half of the book thinking, “Where’s Ella? When am I ever going to find out about Ella?” instead of paying attention to everything that’s happening to Cecilia.
Why did it take you ten years to finally write a sequel—okay, “companion book”--to Just Ella?
I was busy writing other books (Twenty other books, to be exact--I wasn’t just goofing off!) I had really enjoyed writing Just Ella, and I always had it in the back of my mind that I would want to write more about Ella at some point. But I hadn’t really figured out what I wanted to write about her. Then I got the idea for Cecilia and her dilemma a few years ago, and I began to see connections between the stories. When I was a kid I used to imagine characters from various books getting to meet each other (I always thought Anne of Green Gables, Jo from Little Women, and Laura from the Little House on the Prairie books would get along really well.) So this was a way to match up my own characters from different books.
Why isn't Ella the main character?
Because this is much more Cecilia's story than Ella's. Cecilia is the one who has to deal with the most challenges and change and growth. But I did have a lot of fun bringing Ella in, and showing what she's been up to since her book ended.
Will there be any more books telling about Ella and/or Cecilia?
I have thought about it. It has even been suggested that I could do thirteen more books--one for each of the other princesses. I don't think I would ever do that. But I have thought about doing a third book linked to Just Ella and Palace of Mirrors. I could see it focusing mostly on Desmia, and possibly including a trip to Ella's wedding. But I don't have any immediate plans to do that, since I'm busy doing other books right now.