As soon as I finished 'Ghost Song', I wanted to write a third book in the series.
I felt that in the first book, The Ghost Drum, I hadn't developed the character of the mad czar as much as I'd intended to, and as I'd always wanted to be a mad czar when I grew up, I really wanted to give him some space.
This resulted in the third book, Ghost Dance. Whereas the first two share a character in common - the bear-shaman, Kuzma - the third book stands alone, though set in the same world.
It's also, perhaps, the darkest and most sinister of the books.