It’s well-known advice for self-publishers to write books in a series. The reasoning is simple. If one book of a trilogy sells, they all will. Running an ad and taking a loss on one can still be profitable on sequels. And of course, book one can be permafree.
However, the familiarity of sequels runs deeper than a sales tactic. Let’s make a comparison in the visual medium. A movie will need to introduce its characters, stage a conflict, and move the plot to a satisfying resolution, all within one sitting. But when writing in a series, with a model similar to television shows, you have a lot more time to fall in love with characters and their quirks.
Readers come into a sequel already knowing what to expect- the author style, the pace of the plotting, the setting, and of course, the characters. That leaves a lot more time for personal development and interesting asides. A powerful connection is born in readers after, say, book two or three. If they get that far, why not go for twenty?
The author side of the coin is interesting as well. Since I am just now starting to write the sequel to The Seventh Sons of Sycamore, I am realizing what I’ve already gained by not starting from scratch. I’m familiar with the locations and the base players. I know the attitude to shoot for. And it’s so much easier the second time around. Think about it: all the time spent creating character backgrounds and histories is already done. Sure, I need to introduce new settings and people, but you get my point.
A series becomes more than its component parts. All of a sudden there is a saga. A real world. A place to escape to. The author has created more than a book. It’s a legacy. And that’s what many readers are looking for.