Yup - Frank Herbert died before the later books were finished. Later on, a series of new books, both before and after on the timeline, were produced with ghost writer assistance, in cooperation with the family, culled together from Herbert's archive of unpublished Dune materials, and ghost writers filling in the gaps, tying the stories together. Some are better than others, but the main big picture themes are well preserved, in my opinion. To date, still the best science fiction ever written.