So how's the next book coming on, Mark?

Well, it's written, and I am now deep in the rather lengthy editing process, which for me takes longer than the actual writing. I tend to worry endlessly about issues of grammar, sentence construction and consistency, that collectively make for slow progress. But it's getting there. All the drawings are complete, the background material is written, and I am more than half-way through the edit of the story.
The next book is a prequel to Below Mercury, which is perhaps an unusual sequence to follow, but I wanted to go back and explore the early history of one of the characters, before going on into a sequel. I've roughed out what happens in the sequel as well, to try to avoid any serious inconcistencies through the series.
Better get back to that editing ...