I'm feeling pressure with my book reviews, because I've got over thirty Sweet Valley High books available on Amazon Kindle Unlimited. My subscription runs out in June or July, so there's a time limit! I think it comes out to one book every two days, so I can get to them all? I'm not sure.
The California Diaries series is also on Kindle Unlimited. That is a spinoff of the Baby-Sitters Club series. It's the "mature spinoff for older readers", which seems just as common as the "lighter spinoff for younger readers". That is, Baby-Sitters Club, Nancy Drew, Hardy Boys AND Sweet Valley High all have both spinoff of those types.
California Diaries never caught on, for two main reasons. First, the timing was off. It premiered so close to the end of the main series that they didn't have much time to get many books out. Three books per narrator, that's all. The same problem occurred with Nancy Drew On Campus and the one Nancy Drew manga series.
Second, the series is handwritten. As in, the font they used looks like an eighth-grader's handwriting. I had a hard time reading the one book from the series I own, just because of that. I don't know if other readers had the same problem, but that was a major downside for me. If it is difficult to read the book because of the handwriting, I'm less likely to read it!
Thank goodness I've got the eBook version. That has the downside of editors who did a sloppy job getting the text into eBook format--some sentences are cut off halfway through, that sort of thing--but as least I can read all the words.