As an author who's published four books in a still-ongoing series, I can't think of anything more imposing to write than a sequel. Not only do you have to keep track of the details from multiple books while writing it, but you also have to keep finding ways to build on characters and storylines in ways that make sense and feel natural without changing the essence of the narrative too much. And if you botch a sequel, there's a chance you'll also ruin the entire series that came before it.
Even the mere structure of a sequel can pose a lot of difficulty for the writer. When you get right down to it, most followup stories are at a disadvantage from the start because their foundations exist in an entirely separate story -- the story that they're a sequel to. Readers typically won't fully understand or appreciate everything that happens in a sequel if they don't start with the first book in a series, so how does the writer get around that?
The most obvious solution, which I want to discuss today, is by reexplaining the premise of the series in the sequels.
I'm sure we've all read at least one book sequel that pumps the breaks in its opening chapters to remind us of who the characters are and what the series is about. It's one of the most aggravating things you can possibly write; you're trying to get on with the story of your next installment, but every time something comes into play that was introduced in an earlier book, you feel obligated to stop and recap what it is, and that just kills your momentum. Even if you clearly number each book on the cover, you still have to assume that most people aren't going to read the whole series in order or remember every detail throughout it. Some might argue that this shouldn't have to be the author's problem, but catering to your audience at least a little does help both of you in the long run.
The question then is how extensively you should try to reexplain things in a sequel.
A lot of factors weigh on which of these extremes you want to lean towards. The age of your audience, the number of books, the length of each book, and how closely the plot of each one ties in with those of its predecessors are just a few. However you decide to do it, the important thing is to keep the recaps as brief as possible. Otherwise, you will kill your momentum and potentially lose readers' interest.
I posted a survey on Facebook a while ago to get some opinions on this subject. The comments ranged from giving a refresher whenever necessary throughout each book to only doing it in Book 2 and then leaving it up to the reader to figure things out in all future installments. One comment even suggested tapering off the recaps over the course of multiple sequels until you reach a point in the series where the readers are most likely fans who don't need reminders anymore.
The strategy I've chosen for my own series, The Rokshena Revolution, is sort of a combination of all these ideas. The overall story is made up of three separate plots that lead directly into one another, and each plot in the "trilogy" has actually been split into two books because of the length. As a result, I give increasingly smaller reminders of things as they come up throughout the series, but I only do it in every other book.
This is obviously a unique case. However, I do think the tapering idea is a good middle ground to go with. It might even encourage people to start reading your series at an earlier point than they might have otherwise since they'll get a more extensive backstory the further back they start.
In any case, the key to all of this is building a strong rapport with your audience. Getting into a series can be a big commitment, almost like a friendship, so much like you would with a friend, you want to show the reader that you're aware of them and help them out every once in a while. If you're inviting and willing to put in the extra effort for them, chances are they'll be receptive and put in the extra effort for you.
