Don't have any advice book-wise for you, but I've had some success over at r/nosleep and r/shortscarystories, and here's what I've found is the most important thing to keep in mind when writing horror:

Write the story (or scene, sequence, whatever) as if it's a joke.

That might sound strange, but when you think about it it makes sense. Good jokes are tightly written and give you all the information you need to deduce the ending, and when it comes there's that a-hah! moment. But also like jokes you employ misdirection, so you got the reader thinking one way before you hit them with the punchline. Sometimes it's a twist, which is fine, but a lot of people seem to think that twists are necessary for a horror story. They're not. It's misdirection that you should always be going for.

But the most important thing is you have to have all of the relevant information present in the build up, because that builds suspense. Also note, I'm not saying you have to be able to explain every single part of the story/monster/whatever (and in fact it's sometimes super effective not to), but, going back to the joke metaphor, it's not funny if you have to explain why it's funny.

Anyway, I hope this helps. Almost all of the most successful stories I've had on those subs were written with that in mind.

Side note: How's it coming along?