i'm about to fill in the holes i have in my Dune collection, and read the whole....30ish...book series from start to finish.
i read the original series when i was a kid, and fell in love with it. Herberts son and his partner have done a great job carrying it on.
the story of the Butlerian Jihad is great, and tells you the origin of many of the "schools" the Suk , the Bene Gesserit, the Ginaz, the Mentats... it tells you how the Fremen get to Arrakis, and how the feud between Harkonen and Atreides gets started, all on top of the huge man/machine war going on in the background.
the House books are equally awesome, telling the stories that lead up to the original series.
i haven't read much after the original series, but there are at least a dozen more books, ending (at the moment) with Sandworms of Dune, which is supposed to be an effort to resettle space with sandworms and make the spice available again.
that's my recommendation for me, anyway