That said, when each book in a series tells its own tale with a satisfying ending and when I love the writer and the characters, I’m happy to read every book in a series and to look forward to new developments. Sometimes a series loses momentum after several books and that’s a pretty good time to let the characters ride off into the sunset, live happily ever after (or if you’re of a sadistic bent) fall off of Reichenbach Falls--we all know how that turned out, even Arthur Conan Doyle couldn't kill Sherlock Holmes.
When I came to write Gravitas: Valkyrie in the Forbidden Zone, I had no thought of making it a series. I had wanted to write about alien-human first encounters leading to religion since I first read Carl Jung’s Flying Saucers : A Modern Myth of Things Seen in the Skies. Jung saw the UFO craze as a "living myth" and a religion being formed before our eyes. I was intrigued by the idea that this might have happened often in human history and it night not be good for humans or the survival of the planet.
That was the seed of the book and many other elements arose when I started to write. I was coming off a year of being very ill when I started to write Gravitas and I think the book reflects how happy I was that my brain had come back online and I was again able to spark new ideas and spin stories.
Once the book was done, Debbie Notkin was kind enough to look it over. One suggestion she made was that the Furies should play a larger part in Gravitas. I expanded their role a little and in the process, I started to think about who these creatures might be when they’re at home. The Furies inspire such dread (particularly in men, I think because they are the opposite of controllable women) that I realized Sybil of Valkyrie needed to pay a home visit to them and that she still had some adventures to live through. So Valkyrie on Planet Fury needed to be written.
That’s how series books evolve for me. Your mileage may vary.