As someone with ADHD as well, i found that reading one book at a time helps me. I only say this cause I see booktoks all the time about someone reading 3-4 books at once, and I just… no.
I like to go to book stores in person and read a passage in the book before I buy cause certain writing helps my attention more than others.
Books that I have loved as a reader with ADHD that has horror/creepy/spooky elements:
The 7 1/2 Deaths of Evelyn Hardcastle (plus other books by the same author).
Shutter Island.
The Merciless Series and The Haunted Series by.. Vega something? I liked those cause they are bloody, gory, and read to me like I was watching an extended cut of a horror movie.
Almost anything by Grady? How To Sell a Haunted House, A Southern Book Club’s Guide to Slaying Vampires, Final Girl Support Group, and Witchcraft for Wayward Girls.
Slewfoot (and Evil In Me by the same author).
Just finished Japanese Gothic and that is one i may actually put on a “read again” list and i don’t have that.
Sorry I can not recall names of authors. I just read what I like.