Readers like books series, which means that publishers like them too. There are of course different types - some feature the same characters in different adventures - a lot of police procedurals follow that pattern. Some have a theme that runs though a set of stories - a family, an organisation, a quest. Sometimes individual books are episodes in an overarching and continuing story, a bit like a TV soap.
The question I'm toying with at the moment is 'How do you decide that this book is going to be part of a series?' As a romance writer who insists on a HEA ending - my hero and heroine have to complete their story and make a commitment to each other - I'm old fashioned, so that tends to be marriage - and so my stories tend to be stand-alones. The first and third options are not really going to be my thing. To tell the truth, as a reader I tend to avoid door number three as well. I don't go for cliff hangers. Which leaves the middle choice. Now that I can do, and will be, when I finally get the academic stuff put to bed, with the Riviera Rogues, which are kind of adventure/mystery romance.
That one began because I really enjoyed spending time with Cassie and Jake, was reluctant to let them go, and the potential of the detective agency which Jake takes over gave me some scope for further stories, each with a new central couple, but with the agency as a background. It gives me a chance for a new love story and to let characters wander through each others stories. I have Nadine and Ryan and Lisa and Mick lined up, with Michelle waiting in the wings for her story. After that, who knows?
I have an idea that has gelled for a much more romantic series around members of a book group who are starting their own businesses, and lots of ideas for romantic suspense. That plan has an overarching background of a private security agency and sets of books in threes with groups of heroes who work together. I have one that is kind of a stand alone within the series, which I am now wondering might form a line around another theme, probably code breakers of some sort.
All this is very fine. When is someone going to invent the 48 hour day?