If your story becomes a series, remind your readers the fundamentals, such as what the characters look like and any important backstory information. This benefits not only those who read the earlier books sometime in the past, but helps those who start with a later episode.
As an author, your series is one continuous story, but it’s unlikely your readers will read all books in sequence, especially if they’re released at different times. You’ll want to draw them back into the story and plot as quickly as possible so they’re comfortably established. Also consider that your books may not be read in order. Thus, including a plot summary here and there or flashback to a previous story is essential; a prologue is another possibility.