Chapterhouse Dune was the last Dune book that Frank Herbert wrote and some people read the final chapter as Frank ending the series - his wife Beverly was a key part of his Dune writing process and to loose her must have been pretty tough. Instead of writing a seventh Dune book Frank chose instead to write Man of Two Worlds with his son Brian - this may have been his way of "getting his priorities straight". With this in mind D&M as Frank and Bev signing off on Dune works on a number of levels.

But... the fact remains that he had signed a 3 book contract with the publisher. If circumstances had been different there would have been a seventh book. This is the point where someone pops up and says "But Brian Herbert and Kevin J. Anderson used Frank's outline for Dune 7 to write Hunters of Dune and Sandworms of Dune!"

And that is the point I call bullsh!t: In Hunters and Sandworms Daniel and Marty are revealed to Omnius and Erasmus, the thinking machines from their Butlerian Jihad books. These two characters are Brian and Kevin creations not found in any of Frank's notes - that's from an interview with the authors.

In addition Hunters and Sandworms go against much of Frank's core philosophies and at the end Duncan Idaho is revealed to be the Ultimate Kwisatz Haderach... which kind of ruins what FH was building up to (to free the universe from one man's vision).

Spoiler alert - Chapterhouse is pretty good, don't bother with anything written after it. It will spoil your enjoyment of FH's wonderful and epic series.