I started my career believing I'd never write more than three books in a series, but I soon found out that publishers and fans always want more than that. When I began writing the Sleuth Sisters, the demand for "the next one" was great, and the ideas kept coming, so I did seven books. When I finally ended the series, a reader I know personally said, "Good. I was getting tired of that series." (Huh. Did anyone twist your arm to make you read them?) Anyway, my original point was that I believe a series has an arc. If a writer sees it as going on and on, great, but for me, it's pretty short: Introduce the main character(s), see them grow through their adventures, toss in some secondary characters who are fun, and solve a new crime in each book. While readers look forward to the next adventure, for me a series begins to feel repetitive at some point. How many times can the irritating character be irritating in new ways? How many daring rescues must the main char...