Good info….it looks like there will be a 3rd movie, right?
Yes, they are working on a script I think. That movie will (reportedly) be based on the second book in his 6-book series, "Dune Messiah". My understanding is that he wants to do the movie trilogy and then call it good. That should cover the general story arc for Paul, covering his ascent to being the most powerful emperor ever seen, but also the reason he was so afraid that his visions would come true.
The Dune world is rich and deep and fairly fully fleshed out, with politics, religion, science, human nature, planetary ecology, war, etc. It goes much deeper than a movie can portray. Dune Parts 1 and 2 left out entire segments of the book, including CHOAM, the Spacing Guild/Navigators, the Landsraad, most of the discussion of the Mentats and Suk doctors, all the benefits of spice to the empire and people, more detailed discussion on the Bene Gesserit, etc. Basically, it stripped down a lot of what gave the book depth to create a story that could "breathe" on film, instead of the overstuffed 1984 movie (that was pretty bad as a "movie" but in some respects followed the book details better). Unlike the book, they also didn't introduce Alia, the unborn sister of Paul, as a young child at the end of Part 2.
After the first couple books things get really weird, including Paul's son essentially turning into a giant sandworm, the ability to grow clones of humans in alien female bodies, Face Dancer aliens that can mimic/copy someone's looks/voice, etc. And the books can get very long-winded, as Herbert gets deep into people's thoughts, motivations, etc.; it can be a slog to read through despite some of the more
interesting aspects.
Frank's son, Brian, teamed up with a writer and all of Frank's notes about the history and future of the Dune universe and wrote about another 20 books that take place predominantly before the events in "Dune", but also during and after. Those books are not as "dense" and are much more accessible to the reader. Frankly, they are written in a way much easier to digest and would be more adaptable to the screen. Some of the early books regarding Omnius/Erasmus, Agammemnon (the Cymeks), the Butlerian Jihad, the Machine Crusade, etc., might make great movies more in line with "typical" action sci-fi flicks (think things like Terminator/T2, anything with giant mechs, a known-universe wide jihad of religious wars, etc.).
See here for more:
Dune (franchise) - Wikipedia