I'm a plotter overall, especially with a series like Forgotten Worlds, but I allow room for surprises and flexibility to change things as necessary. No idea is perfect in its planning.
That's why as I've been writing Book 15, I've been revisiting Books 13 and 14. I left something unfinished in Book 14 which gives me a good subplot to wrap up in Book 15. And that also gave me a new idea for an extra end scene for Book 14, which teases a big question that could make things more complicated for me but add tension to the overall series.
While I try to keep each book as a standalone episode with its own beginning, middle, and end, I also have the overall series arc to consider. There are some things that get carried on for some time before being resolved, as readers have now discovered. My notes are long and complicated, and it's getting to the point that I may need a better format for tracking all these connections. I just don't have the time to sit and make up charts of what happens when and what else it affects, nor the inclination. I have too many other things to do in my day than to reorganize my notes.
Right now, I am happy to report that Book 15 is up to 7000 words. Considering I broke from it yesterday in a sidetrack of ideas that came to fruition in an extra scene at the end of Book 14 to tie into what developed for Book 15, I've still maintained a decent pace of writing. And this just re-emphasizes the need to stay two books ahead in writing of what I'm publishing.
The biggest challenge is always to keep the stories exciting and interesting and largely complete episodes with lead-ins to what's to come.
It's getting more exciting with some major events (and HUGE revelations) to come!