@P.B. Lecavalier - publication order.. mea culpa.
My mistake - as I said, it is a long time since I read these!
I seems I've mixed up the order completely. So: all of my previous comments actually refer to one or other of the last two books (which would be "Heretics of Dune" or "Chapter-house: Dune"); the author's introduction I was referring to was also from there.