Re-introducing the characters from the first book, adding brief reminders of what happened in the first book, re-describing some of the places, etc. Seems to me that's a pre-emptive move to be sure readers can enjoy the second book even if the first one isn't on the bookstore shelves...
I agree. The first two books in my series can stand alone. You could read one or two and still feel somewhat in the know for book 3. Despite that, for every book I insert reminders throughout on certain things to help clarify - just in case there's a new reader.
Though I had a reviewer who started with book two say there were definitely some things she felt in the shadows about. I don't know that they can be helped totally. I mean it's a series book, after all, and you're writing to appeal to a reader who likes to delve back into familiar territory. Re-hash too much and you turn the regular series reader off, IMO.
A little OT - but I like this discusson...
I was a Patricia Cornwell fan for years. But by book 5 or so, there were certain repeat characteristics and reminders that drove me nuts. It was like - Yes, yes we get it she loves to cook!!! After awhile it came off as too much telling, even though I realized she did it for the new reader to the series. Another thing that drove me a bit nuts - I never felt her characters grew. From book one to book seven, her adult characters never "gave" in on their most stubborn traits.
I love that in YA it's about the characters changing based on experiences. Cornwell wrote as if adults can never change. And that's just not true. Yeah, it may take a lot to make us change, but we change!
But I digress. I was going to say that one and two, of my series, are stand alones and were actually written as such. But I've always felt that books 3-5 are for the "fan" of the books - even with the reminders etc... If you start with three, I think you could follow the action fine. Where I think having read the previous books helps is with the character's motivations. As a new reader you might feel some of the actions don't ring true, simply because you haven't been on the journey the entire time as that character's well, character developed.
What I'm trying to have faith in, is that someone will pick up any of the books, not get frustrated that they've "come in the middle" and be willing to go back and read the earlier books. Though, in all honesty, the series takes the characters from 9th to 12th grade. So there's a good deal of maturing that occurs and ideally the reader will take that ride with them in order. Ideally!