(1984: fifth book in the Dune Chronicles)
From the inside flap
". . . the planet Arrakis--now called Rakis--is becoming desert again. The Lost Ones are returning home from the far reaches of space. The great sandworms are dying. And the children of Dune's children awaken from empire as from a dream, wielding the new power of a heresy called love."
Heretics is a good, slow-build read. Leto II has been dead for thousands of years. The Bene Gesserit and the Bene Tleilaxu are still engaged in power struggles with each other whilst squelching insurrection within their own ranks. Other groups, including the wild and sexual Honored Matres, have entered this cautious lead-up to war. (The Honored Matres are intent on supplanting the Gesserit Sisterhood and the Tleilaxu.)
Not only that, a young girl (Sheeana) -- a possible descendant of Siona, who helped bring Leto II down -- and a recent Duncan Idaho ghola are showing signs of rebellion, whom the Gesserit and the other groups must control or kill.
Heretics has some interesting characters, Herbert's usual epic-minded writing and potent, series-changing twists, making this a worthwhile entry in the Dune series. For Dune purists who love the Atreides storyline but not the other group politics, I would suggest borrowing it from your local library first (if you are so inclined).
Followed by Chapterhouse: Dune.