The Hourlings Podcast Project, Episode 13, is all about writing a series of books. It's very popular to do series in both Scifi and Fantasy.
This Ep's contributors:
Martin Wilsey - https://www.martinwilsey.com/
S.C. Megale - https://www.scmegale.com/
David Keener - http://www.davidkeener.org/
Jeffrey C. Jacobs - https://www.timehorse.com/
Erica Rue - https://www.ericarue.com/