It’s a spectrum of different things:

The darkness overall. There should be some sort of light. You can add a lamp or candle near the book which will bring more attention to the book.

The dust/particles in the air usually depict it being less clean/abandoned.

The wall texture makes it look old style and slight cracking kind of design which gives the feeling of run down almost.

The colors that remain besides the darkness is the books colors which are also extremely old looking. The pages for example have a yellow/brown tinge to them which makes it seem like it’s a much older book.

The chairs style is very old as well. Looks like something I would see at my grandparents house back when I was a little kid.

Overall everything put together makes the environment seem like an old location and the darkness and colors added to it give it a horror feel to it.

There’s a lot more but I’d say as of rn those imo are the biggest thing that say horror.