yeah, frank herbert wrote 6 out of the planned 7.
children of dune
god emperor of dune
heretics of dune
his son wrote 2 trilogies of prequels
Battle of Corrin (planned for September 2004)
overall, the two prequel trilogies are really good. dune is amazing, but the sequels go downhill. it talks about reanimating people through cloning and other weird stuff. i read the whole series, and the last 2 are accually pretty good.
hope this helps