As the author, a story that becomes a series is usually thought of as a single book. Readers, however, may not read them in order, so be sure to include relevant backstory information so they can follow the characters and plot.

This is something to watch for in your final edit. Ask yourself if occurrences in a previous story need to be reiterated for the current story action to make sense to a reader who hasn’t read the previous book. Such recaps are also appreciated by readers where significant time has elapsed since reading its predecessor. If your books are released years apart, then this is a given.