Yesterday, I briefly mentioned one dilemma I face when I'm writing a series, and that is how much information from a previous book to include in the book I'm currently writing.
The short answer is, as little as possible. For one thing, I don't want to bring the pace to a screeching halt while some Basil Exposition relays the plot of the previous book. For another, I want every book I write to "stand alone" -- I don't want readers to feel lost if they pick up a later book in the series.
That's why I usually move the action of the current book away from the other books in the series, to a different castle or town. That way, the main characters of this book aren't meeting up with the past characters, although, as in the current w-i-p, they may think about them sometimes. But those thoughts also have to do with the backstory and motivation of the current characters, especially if we're talking family members. Even so, I try to keep them to a minimum, because those previous characters have already had their story. Now it's the "new" characters' turn to be in the limelight.
That said, I do want readers to think, "Oh, there was a book before this? About some of the same people? I've got to buy that now!" Sales mean I can keep writing, after all. So I'll sometimes have the previous book(s)'s hero or heroine or both make a cameo appearance, usually at the beginning or end of the new book.
In some ways, writing sequels is easier -- I've already "met" the main characters -- but finding the line between what a reader needs to know from a previous book and what you can/should leave out can make it tricky, too.