3.5 stars.
There is a certain specific joy to reading a big fat multi-character horror novel and I AM BEHIND YOU falls firmly in that sweet spot. You can get lost in it in a way it's hard to get lost in other kinds of books.
I found Lindqvist's most popular book, LET THE RIGHT ONE IN, underwhelming. (The Swedish film was better, imo, pulling out all the good stuff and getting rid of the bad stuff.) Here we hop from character to character, getting to see the events from different points of view, without too much repetition. Even with 10 characters (12 if you count the animals) it's easy to keep everyone straight and there's a nice throughline of the story, with good juggling to make sure you don't get too much or too little of any one thing/perspective. There's a steady increase of tension with new discoveries/obstacles every so often to keep things moving. And, of course, there's inter-character drama as things get more and more stressful.
Lindqvist's scary stuff is unique and distressing and something you can really picture in your mind's eye, he's quite skilled at it. And the character work here is also very good. You really get a feel for who all these people are, and enough of their backstory to feel fully fleshed out.
It is funny to me how many of the reviews for this book complain about the ending. This is very much a symptom of how horror readers come looking for certain tropes and structures. Horror is a genre with notorious third act/resolution problems, and here we have a book that actually handles it pretty well but it bothers people because they want their bad third act with a little bow that doesn't actually wrap up anything. (Example: Yes, you can say "oh we have explained it, this all happened because this monster wants XYZ," but if you actually say, "but WHY?" almost no horror novel can back it up, it always involves just accepting the supernatural, just not as much as this book does. We could use more practice at it, tbh.) I found the ending perfectly in keeping with the rest of the book, but if you didn't love THE RUINS or other books that don't give you a tidy little bow, this may not be for you. (This book does have a lot in common with THE RUINS structurally, if you liked one you will probably like the other despite their drastically different settings.)
Note: this is very much an example of Straight Man Writing. All the straight men think about sex a lot, basically no one else ever does. (It often seems that no one else except the straight men even care about sex at all, to the point that it feels almost homophobic with one set of characters.) It is not the worst offender, and it's certainly common in the horror genre, but something to keep in mind if such things annoy you (as they do me).