Previously, on Princess Trek…

So I'm working on book four of a series, and I'm struggling with is how to provide all of the background information.  I've now got 300,000 words worth of "what came before."  Not all of that information is relevant to the current book, but some of it is.  So how do you work that in?

There's the "Our story so far…" approach, where the author presents a prologue that sums up the previous books.  I can see where that might be useful in an ongoing story, like book four of Lord of the Rings or...