I just read an interesting article that was brought to my attention by the Romance Writers of America e-newsletter I receive. It was about author Robyn Carr and how she found success, after starting out writing historical bodice rippers, by writing a series of books that take place in a small town of 600. Her books focus on different "alpha males" in the community and allow character to reappear in different stories. I remember romance writer Jennifer Crusie doing the same thing with three of her books. It's a great idea. People get to know a character, and then they get to see him from a new angle. Today's romantic lead can be tomorrow's protective brother, etc. . . . Plus, it's like watching the "Andy Griffith Show," for all those who've seen the popular 1960's TV show that took place in a fictional small town in North Carolina. The reader becomes familiar with, and hopefully likes, a cast of characters in a small town, and gets to revisit with them in other books. That's why book series are so popular, as opposed to one-offs. People like to spend time with characters they already love.
Here's a link to the article about Robyn Carr.