In my case, I wrote Book 4 of the Daoine Maithe series because my daughter demanded another installation in the adventure. Going back, I wrote the first three books because the story blasted into my imagination and I just had to write it down. Once I started writing, the story itself led me off in all sorts of different directions than I originally intended and it exploded out into an epic adventure. I thought I had finished that series as a trilogy, but my daughter would not let me rest, repeatedly asking for the next book. So in fact, I wrote Book 4 to keep her happy. Hope everyone else likes it too.