I see a lot of negative reviews for different reasons so here's my thoughts/replies and honest opinions on all of it, WITHOUT SPOILERS.
1) Many people are complaining that it's too political. Personally, I think King did a fantastic job at recapping all of the different things that we all heard during the beginning and height of the pandemic. The book takes place in 2021, and we were still hearing ALL of the things he mentioned in 2021! And he made sure to hit both sides of everything. From the Trumpers claiming it's a fake virus, to those on the other side of things that wore a mask everywhere and got vaccinated. He showed exactly how 2020 and 2021 went through a fictional story with fictional characters. Although King DOES give his own opinion at the end, in the author's notes, he really just showed both sides of everything in this story. 2020 & 2021 were very political years, so I don't think anyone could write a story taking place in 2021 WITHOUT bringing up covid or the political sides of everything. Because even though covid was not political, many people made it so.
2) People also complained that this is not the shy, timid Holly that they remember. I read the entire Bill Hodges trilogy in order, RIGHT before I read Holly, so I think it fit perfectly. Her character develops a lot throughout the trilogy and by the time we get to her in her own book...I thought everything fit her character appropriately.
3) I loved the story! I enjoyed following Holly around on her investigation. I guess my only complaint is that the ending was a bit predictable. I saw it coming. But I still enjoyed it!
4) YOU DON'T HAVE TO READ THE ENTIRE TRILOGY BEFORE READING THIS ONE! Sure, it helps and is probably recommended so that you can follow Holly's character development from a shy, timid woman who was still under her mother's thumb to an independent woman who faced her fears and realizes she's much stronger than she thought. But, he DOES briefly recap what you need to know from the other 3 books. It is a BRIEF recap though. What he failed to recap, however, (and I had to Google) was Barbara's run-in with a man named Chet. I got the impression it was something horrible that happened but not sure he ever said what happened exactly? This is from King's book "If it Bleeds" (which I have not read.) So, I was a bit lost on that. I DO wish that he had given a bit more of a recap on all of these things, but in the end, I still felt that you could read this, and enjoy the story for what it is, without reading the others first. (If you have the time, I do recommend it though because the entire Bill Hodges trilogy is fantastic.)