The Mr Mercedes trilogy is a thriller series not horror, very good though.

Koko by Peter Straub is the only book I've ever read that made me turn the lights on reading it, scared the shit out of me.
This is from goodreads reviews.

Just finished book one of this trilogy. This has been a group read with The Eclectic Club.
I really don't know what to say here. This book ****s with you. It leaves you feeling dirty at times.
Was I confused?...several times! A lot became clear but there were still many paragraphs and sentences that read like the mad dribble you'd hear at your local nut house.
But did I like that confusion...that mad dribble? I sure did.
This book was totally ****ed up but I enjoyed it enough to give it 4 stars and to buy books 2&3 of the trilogy (Mystery and The Throat).
You can definitely see why King and Straub collaborated when you read Straub's writing style in this book. They are on the same plane here, but IMO, King still comes out on top!
If I ever see a playing card with the word "Koko" written on it...I'm going to get the hell out of Dodge!

https://www.goodreads.com/book/show/19577.Koko