Book 2 is grim and deflating, 3 picks up the pace again but is really just setup for 4.
After 4, Herbert’s wife passes away and he takes up with his much younger editor. From then on, the books get significantly hornier. YMMV.
I never realized that the change in tone in 5 and 6 were due to a new editor. Interesting, but it makes sense.
I think 5 and 6 were ... fine. But I have trouble pinpointing what they were really about. There is a lot of movement but no direction.
I hesitate to use the word because afaik Herbert was a widower at the time, but can’t think of a better one that hits the right tone.
Affair: a sexual relationship between two people, one or both of whom are married to or in a long-term relationship with someone else. "his wife is having an affair". (Google)
Websters: “a romantic or passionate attachment typically of limited duration”