Aimee says that kids by nature are collectors, and this applies to books as well. Series books tend to be for kids more so than other types. She points to one of her favorite series while growing up: The Babysitter's Club
When it comes to series writing, Aimee says there are elements to every successful series:
A solid hook with endless possibilities
Characters you'll want to revisit again and again
And an overall series arc/theme that weaves in naturally with each title.
If you're going to set out to write a series, you'll need to do some research first. There are no hard and fast rules, but to understand series and packaging, you should READ. She says it's the best way to learn the genre.