Thanks so much for the email. The third book in the series is complete -- it is tentatively titled The Man with the Silver Nose. I really like this book. It introduces new adventures for Colophon and her brother at The Tower of London, Windsor Castle and Westminster Cathedral. The decision, however, as to when (or if) to publish it is up to the publisher, Houghton Mifflin Harcourt. If you are inclined to do so, please email the publisher at email@example.com and let them know that you would like to see the series to continue. It is very important for publishers to hear from readers -- it really makes a difference.
I do have another book coming out in 2017 (which seems SOOO long away) — it is a mystery based on the artist Vincent Van Gogh. Keep an eye out for it.