The most interesting thing that comes to mind with the book now, is that it's mostly from the perspective of Major Briggs. and seeing the ways that The Major factors in to the world of the series now puts a lot of the book into a new light. He would have had reason to hide or purposely misreport details about things. so that unreliable perspective could be a clue. a lot of timeline inconsistencies appear between the book and the show at the points where they overlap, and now that the timeline of the show has been blown completely wide open those inconsistencies could actually be on purpose.
I have no idea what the 2nd book will be. Obviously it can't be another thing written by The Major. Maybe it'll be written by cooper and shed light on his alternate reality or something. That would probably be too easy though.