Julia Spencer Fleming is one of my favorite mystery authors and I found this on a blog where she was talking about how to keep a series fresh (she just released her 8th book in her series)

Write about issues that genuinely interest you. I tend to write about literal issues: migrant workers, vaccination, corporate malfeasance. It takes me a year (or longer!) to write a book (sorry, everybody) so I need to find what I’m writing about compelling. However, a story doesn’t have to be ripped from the headline, to quote L&O. In Through the Evil Days the issue is how to deal with a surprise pregnancy, and, more deeply, what people will and won’t do for their children. Of course, I’m getting older, so look for my eventual novel based on a scathing denunciation of Denny’s Senior Special.

And should you want to check out her books,  the first one is In the Bleak Midwinter and the most recent is Through the Evil Days.