Magic Below Stairs
is a companion novel to the Cecelia and Kate books by Patricia C. Wrede and Caroline Stevermer. It has a slightly younger target audience, and it is not written in a letter/journal format the way the Cecelia and Kate novels are.
I definitely want to see more of Frederick, and I really hope that Caroline Stevermer continues writing stories about him. The ending of the book definitely seems to leave that possibility open.