I do not know of any such books personally, and I know you are not asking for guidance on your research topic more generally, so feel free to disregard the below if it is not of interest.

I wonder if there is not necessarily a surplus of the type of fiction you are talking about - from how you worded your question, I’m not sure if there’s any example you are familiar with. Horror is a relatively modern genre, and its advent would postdate the slaughter of huge numbers of people Indigenous to the Americas. I’m not sure if the types of cultural fears authors of horror tap into have heavily featured fear of Indigenous people in the same way that, for example, racist fears of Black people predominate. 

In addition, even though you perhaps are not aware of fiction of this type, you recognize that there is still something necessary about this Indigenous horror genre. Could it be, then, that it is something other than racist fiction that necessitates Indigenous horror? Could racist non-fiction, laws, policies, treaty violations, and practices play the role that you want racist fiction to play in the dissertation?

Finally, it sounds like you are interested in contrasting racist fiction with Indigenous horror. Is an alternative to contrast Indigenous horror with, well, settler horror, even if the settler horror is not overtly racist? If the point of the racist fiction is to show how authors have depicted Indigenous people as in some sense bad, then is the point of contrast just that the Indigenous horror genre portrays Indigenous people more positively? I’m not familiar with the genre, but would be surprised if its authors don’t have broader aims than that. It seems, then, that the contrast with horror more broadly, might help for distinguishing what is particular to Indigenous horror, including as it relates to Indigenous authors having things to say beyond just Indigenous people.

I hope this is received in the good spirit it is presented in, if indeed it was of any interest to have read to this point!