I'm bringing something "new" to the table with my book series. What I'm calling the "interlogue". Let me explain.
Novels on occasion have a prologue, which happens at the beginning of the book. There's also the epilogue, which happens at the very end. This is known, but I'm telling you this to put things into perspective.
The interlogue is something similar, except that it happens about in the MIDDLE of the book. To be worked out if it will be in the exact middle of the BOOK or the STORY, however, but you get the idea.
This will be a random, one-off chapter which has nothing to do with the book you're presently reading...but has significance to the SERIES and most likely the final volume. It'll serve as an intermission and provide clues, make you think about what's to come; as well as the significance of other events that are happening 'behind the scenes'.