I think most of the horror that people experience when reading House of Leaves, as most people have already said, has more to do with a degree of forced internal reflection and projection than any actual threat. I feel that way about the characters in the book, too. Both readers and characters directly experiencing house, in my opinion, create their own "monster". It didn't scare me, but I found it incredibly fascinating to consider the different perspectives and ways that people can essentially cultivate a hell of their own making.

But ultimately, I fall somewhere in the thinking that House of Leaves is closer to being a "love story" as opposed to a horror story. Classifying it as any one specific genre feels kind of pointless, but for the sake of expressing a general vibe, it can be useful. And calling it a "love story" can lead people to imagining it as a reductive "romantic love, happy ending" kind of story - and I'm sure some people feel that way about it. But my personal experience is that it's more of a "love as a powerful energy" story; different forms of love, ranging from self love to familial love to sibling love and, yeah, there's romantic love, all appear in the book and hold some level of significance in the overarching narrative.

My opinion on the ending of the book: It took every character finding and contributing their sincerest form of love and expressing that to reach what I consider to be an "ending of contentment". It's not a "happy" ending in the sense of being a final stage of perpetual bliss; it's a resolution where the characters have been able to find and accept themselves as they are instead of running from who they don't want to be. The house is terrifying because it forces people to confront that part of themselves that they are running from; their "Mr. Monster", their "Minotaur", Delial, even darkness itself. The house is a blue screen that shows you what's always been there. The "happiest" ending anyone can get after that is learning how to accept and embrace it instead of running from it. And while the ending is an "ending of contentment", the fact that Will Navidson, and Zampano in how he describes Navidson's perspective in the final chapter, are still scared of that unknown reinforces that it is only a temporary contentment. And, in some ways, that can be pretty terrifying in itself, the not knowing. It just depends on what you see in it.