If my editor buys my second book, then my first two published books will be connected books rather than standalones. The heroines of these two books are sisters, both with paranormal gifts that affect the plot of the stories. There's a third sister, but I currently have no story for her. It may just end up being a two-book series.
I have another completed manuscript that I think may eventually sell, but it's not connected to anything I've written before or anything I've got in the works. I suppose there are secondary characters in the story who could warrant their own book, but I haven't really thought toward that. So it may turn out to be a true standalone.
And the WIP I'm working on now is planned to be the first in an ongoing series about a private international security and threat assessment agency made up of former government agents from a variety of agencies (and even countries) who all shared a common past experience--a deadly terrorist incident in a fictional central asian country. This experience will in some way inform and affect the characters and their stories in each manuscript.
Personally, I love interconnected series. I'm a big fan of Gayle Wilson's Phoenix Brotherhood, Deb Webb's Colby Agency books, Dana Marton's SDDU series, etc. But do readers get turned off by series, fearing that if they come in on things in the middle, they won't know what's going on?
So, what about you? Do you like series books (specifically in category lines) or do you prefer standalones? If you're a writer, which do you prefer to write?