Readers love series. I love series too. I've done so many that any more I think I naturally have secondary characters who I just know I will be seeing again. Maybe it is just that I hate leaving this place I have spent hours building and people I have come to care about.
This month I finished a six-book series for Harlequin Intrigue called Chisholm Cattle Company with my book WRANGLED. If you are considering writing a series, you need to have something that holds the books together. The easiest way is, if you want to write say a 3-book series, have three siblings or three of anything that connects the books. With the Chisholm Cattle Company, I had six brothers thus a 6-book series.
Another way to "meld" the series together is to have an over-arcing mystery. With the Chisholm Cattle Company, I had the six brothers' father and his new wife be involved in the mystery that began with book one and carried over all the way to book six. Each book stood alone and had its own mystery that was solved by the hero and heroine. The over-arcing mystery advanced with each book.
Another device is simply putting the books in the same town. A variety of writers are doing that right now. I started my Whitehorse, Montana series 24 books or so ago using that device. I also incorporated the other devices to do 5 or 6-book mini series within that framework.
Recently I began another series with my single-title book, UNFORGIVEN for HQN, which comes out in December. That series centers around the fictional town of Beartooth, Montana and the sheriff of Sweetgrass County. By doing that I can move my story around the county and still have characters who hold the series together. When you're writing suspense and killing off characters, it's a good idea to have lots of room for your story. :)
Often a writer will do one book with no thought of making it into a series. That has happened to me twice with the same location. After I wrote CRIME SCENE AT CARDWELL RANCH, I didn't give it another thought. But when my editor chose it for a worldwide giveaway, readers began asking for a sequel. When I mentioned this to my editor, the next thing I knew I had a contract for JUSTICE AT CARDWELL RANCH.
The lesson to be learned here is this: fill your story with secondary characters you can use should you ever decide to do a sequel. I didn't have a lot of characters to choose from. I went with a bad boy brother of my original heroine. I really should have made that family bigger because when my editor wanted more Cardwell Ranch books, I literally had to dig relatives out of the woodwork. :)
I am currently working on a single-title HQN set in Beartooth, Montana. REDEMPTION is set to come in February (If all goes well.) I love seeing my original characters again. To keep things interesting, they have unforeseen problems as do my hero and heroine.
If you're going to build a world and populate it, at least consider making the book into an on-going series. It is much easier to write about a place you've already created. Readers love it. And you and your readers won't have to wonder what happened after they turned that last page. They can find out in the next book of your series.