You know what I love? I love a great series with characters I want more of.
You know what I hate? Series that flog me to death with characters who never develop, even if they were great in the first three, four, five, or six books. Not all long series are like this. I just binged on CJ Box's Joe Pickett series and can't wait for #17. But, to me, this is an exception rather than a rule, and has more to do with the protagonist taking a fairly minor role in the alternating points of view. It's like the 3rd Joe Pickett book instead of the 17th, in page-time, if you know what I mean.
So when it came time to write my own series, I wanted to leave my readers wanting more of each protagonist when I finished with her. I wanted to take a character and develop her over a three-book story arc. Each of her books would be a standalone mystery novel, but when combined with the other two books about that protagonist, they would tell a story about her with significant, interesting personal growth occurring from book-to-book, and a satisfying romantic storyline as well.
With Katie, for example, we have Saving Grace where she's trying to belatedly grow up, Leaving Annalise where she's becoming less self-focused, and Finding Harmony where she learns she can't expect perfection out of others any more than she can expect it from herself. When I wrote the ending of Finding Harmony, I liked where Katie was. Honestly, I didn't want to mess up her world, and if I'd continued with her, I'd have had to farg her up. The way I write, the protagonist must have skin in the game. She has to have all she cares about at stake in each mystery in order for her to care enough to put herself at risk by taking ownership of solving it. My gift to Katie is a long and happy life with her husband and kids.
My gift to you guys is that I don't dilute her impact on you by going on and on and on. (You're welcome!)
Yet I don't want to start over with a new world after I finish three novels for a protagonist. So I chose to do a world, What Doesn't Kill You, where I can stay within the same community of characters and spin off trilogies for protagonists who we already know and care about. Enter Michele, Katie's law school roommate (Going for Kona, Fighting for Anna, and Searching for Dime Box), Emily, Katie's former paralegal (Heaven to Betsy, Earth to Emily, and Hell to Pay), and Ava, Katie's singing partner and island friend (Bombshell, Stunner, and Knockout).
I did the same three-book character arc thing with them.
I let my protagonists make guest appearances in each other's novels. Katie is in fourteen of the novels. Emily is in ten. Ava is in nine. Michele is in eight. Maggie is in five. Laura is in three with three more in the works. And many other characters cross over as well. Collin isn't a protagonist but appears in ten novels so far; Rashidi is in ten; Nick shows up in seven. Etc.
For better or worse, this is my strategy, and it's working for me. I know how hard it is to walk away from a character you love and not continue in-depth in his or her world. Bring on more Joe Pickett, ha ha. Thanks for sticking with me anyway! I promise you'll keep getting more of your favorite What Doesn't Kill You characters.