The Long Middle
The four novels between the first book and the last one Herbert finished are the part of the series most readers treat as an obstacle, and they are the part I would put in front of a newcomer.
The second novel turns a victory into a campaign, and the tone darkens immediately. The third is the strangest of them: a coming-of-age story told through dream sequences, religious imagery and a deliberately alien form of narration, and it is the book where the series stops being about politics and becomes about belief. The fourth follows a boy into godhood and then spends a long time showing what godhood costs him and everybody near him. The fifth, the last of the four, is a return to form: politics, institutions, a factional struggle, and an ending that lets the reader see the shape of the whole enterprise.
What the four are doing is reinventing the religion. The first novel plants a faith inside a character and hints that it might be a lie. The second turns that faith into a weapon. The third takes it seriously enough to write the dream sections at length, which is a decision you either love or you do not survive. The fourth makes it a government. The fifth shows a government of faith being administered by frightened people, and that is where the reader learns that the real subject of the late Herbert-era books was never the hero.
What is fascinating, and slightly alarming, is how much the middle resembles the prequel cycle that came later. Both spend their time inside institutions rather than around them, both are interested in how a plan is transmitted across a generation, and both treat a character's destiny as something handled by other people on his behalf. The prequels were often criticised for being second-hand. Reading the middle directly, I find that criticism too simple.
The middle is also where the series discovers that its own faith is the most dangerous thing in the books. A political threat can be voted on. A faith cannot, and by the fourth novel it has an army.
If you read the first novel and then skip ahead, you will think these books are adventure stories with a religious streak. If you read them in order, you will think they are about institutions and the terrible difficulty of believing anything, which is what they are.