The Nancy Drew Girl Detective (and the Hardy Boys Undercover Brothers series) decided to switch to a trilogy format. No more standalone books! From here on out, every book is part of a trilogy!
This ended up being a bad idea, for two reasons. One, it's obvious that none of the ghostwriters were in contact with each other. It's hard to write a good entry in a trilogy, when you haven't read the other books in said trilogy. Two, the stories were not made for trilogies. They were generally two-book stories, which were stretched out into three books, by means of long recap, filler and repetition.
It seems that Sweet Valley High also decided to switch to a trilogy format, towards the end of its lifecycle! Except it was a "miniseries" format, not a "trilogy" format. The first one is about a serial killer, which ran for five or six books. The second one is also about a serial killer, which ran for five or six books.
I read the last two books in the first miniseries, and oh man. It is Filler City! This miniseries should have been three books, two books if the author was really on task, but it was stretched out into six. I can kind of understand why; they wanted the grand finale to be Book #100. But still, there is no way the story should have gone on so long.
I have hope that the miniseries format won't turn out to be completely awful. The publishers must have gotten the message that "stretch the story out as long as possible" is a bad idea, because afterwards, the miniseries are either two or three books long. After my experience with Nancy Drew Girl Detective, I'm happy about the idea of not stretching a two-book story out into a trilogy.