Assuming that events in this book series (what is it? is it a secret?) take place on a linear timeline—that is, each new book takes place after the ones before it—then you could just have your character be from the latest time you remember and have them not be aware of anything that happens later in the series. It would be like having a character from an earlier era in Earth's history. If they're from, say, the Renaissance, they could be forgiven for not knowing much about what's happened since and holding some beliefs about the world that have now been outmoded. Same goes for a character from an earlier point in their canon, I'd think.

That said, I'd be leery of doing it myself without at least brushing up on major events/changes in the canon. Does it have a wiki? Just about everything does.

~Neshomeh