One of the writers in my writing group, Cinda Chima, has written three related books (check Cinda's stuff out here) and while we were reading the third book for her, she had some questions about how much information to include about what had happened in the prior two books. Her books weren't sequels, like Electric Boogaloo is, but the question is still a tricky one, because you have to assume that a large portion of the readership for the second or third book are going to have read the first one. Since her books were related, but were not direct sequels (meaning each book focused on a different main character, but the books took place in the same world as each other, and some of the characters overlapped - a main character in one book, for example, becoming a supporting player in another), Cinda wanted to make sure that she included enough information for the new readers without boring the returning ones. It's a tricky balance - how much back story to include.
This is the question I will have to wrestle with even more than Cinda did, since Electric Boogaloo is a direct sequel to The Book, not a related one. It picks up just months after the first book ends and keeps on going, and there have been times while I'm writing where I start wondering "do I need to remind readers that S (my protagonist) likes this kind of thing?" or "should I say something about what happened to character A in the first book?"
It's too early for me to start wrestling with these questions, so I don't. I just move on. But at some point down the line I'm going to have to figure out how to strike a balance between the old readers (you know, assuming I have them) and the new ones (again, helloooo assumption). I'll have to take a look at my bookshelf and see how the series I've read in the past handle this situation. You know, months from now, when this becomes an issue. Also, I will have to find a reader who hasn't read the first book, who can judge for me whether or not I've stricken the balance. My beloved writing group will have some great suggestions, I'm sure, but since they've seen the first book, they won't exactly have fresh eyes.
All right, clearly I'm insane, as I'm worrying about an issue that will not even arise for months. This is just another way I'm procrastinating about doing the laundry.
ETA: Jessica from BookEnds LLC has a great post on what you should be writing while your book is on submission here.