Jason asked Mike Carey:

Any chance we'll see Felix Castor again?

Mike Carey I hope so! It's been hard to get that conversation going. At the moment, M.R.Carey sells a lot better than Mike ever did, which means my publishers are more interested in stuff that will go to press under that name. I have plans for the sixth book, but they were drawn up a long time ago. I think I'd have to start from scratch and see how they hold up. I'd also have to think myself back into the right mind set, and get used to Castor's voice again. None of these are reasons not to do it, they're just hurdles I've got to jump.

On another level entirely, I'm a little worried about accidental revisionism. Sometimes when you come back to a series after a long absence, your perspective has changed in ways that become problematic. I did a blog post here on Goodreads about exactly this. If I felt that was happening, I think I'd have to abandon the effort rather than write a Castor novel that felt like a fake or a pastiche...

flag