I have to admit I’ve been behind the game on this one. I just haven’t found a great reason to write a series of linked books until recently. Readers, however, love these. Trying to get clued in on the motivation for this, I recently asked a reader why she liked series and she gave me a “Duh!” look, saying she liked seeing how the main characters’ lives played out. She fell in love with the secondary characters in books she liked and wanted then to see where their stories went.
She has a point.
Before this, I’ve written the first three books in a loosely-linked series, based on friends’ experiences at 
holiday times. I liked seeing how we interact with one another around holidays and I started out with . I tend to be a little off kilter in my perspectives, some times. Having recently been chastised for not jumping further into the series stories, I decided to get with the program. I’m currently writing about three brothers–one an adopted foster brother–who run an architectural salvage yard, the Salvaged Series, previously entitled Blue Collar Boys.
Series books offer a different set of challenges to authors because we must attend to a primary story and give at the same time, thought to fully-fleshed out secondary characters. We have to set them up for their own stories and then we have to deliver snippets of the initial couple into subsequent books.
A whole lot of weaving goes on and it makes me appreciate J.K. Rowlings even more.