The first one - Pursuit was actually originally the 3rd book and the 3rd book Ennead was the first. Pursuit came out of my pot-boiler mind whose first draft came along in 1984 or so. What prompted me -is NOT a stupid question by the way. It's a very good question.
Only one of the immortals has appeared in "Pursuit" the first book, the others have walk-on roles until the 3rd book. The Ennead is at the crux of the story - the raw materials for all other novels.
The central theme is the immortals holding the higher essence of humanity through dark times so they (humans) will remember. Later books deal with ways of resolving conflict peacefully and in other ways too.
Its likely that all nine novels will not appear on Medium. I may stop them at #3. Also there's something about writing long fictional themes. The characters come alive and shape the story such that each novel has a life of its own. And the series as a whole too.
In this series its about the evolution of the characters from adolescent themes to old soul themes.
The nine book series is built on the enneagram as a process tool - "Map to Arc of the Immortals"