The prequels by Brian Herbert are written well and lays out the foundation on which Dune is based on. One of the major points they clarify is the rift between house Atreides and Harkonnen. I won't give it away, but it is steeped in rumor too deep to find the truth. They also explain where their tech came from and the major orders, ie:the mentats...
Didn't Brian retcon a load of stuff - like the Bene Gesserit being telepaths and a load of other continuity issues? A couple of friends of mine are Dune nuts and think all of the Brian Herbert stuff is really bad.
I have read the first 3 in the series and do intend to read the last 3. The first book is one of my favourite books of all time.
Also I have a 1st print 1st edition 1981 Hardcopy of God Emperor of Dune