It's not a horror in the paranormal sense, but a thriller with a rather horrific villain. One thing I've liked about King, regardless of what I thought of a specific book, is that he creates vivid characters. They aren't two-dimensional, and he has the ability to make us feel as though any ordinary person could be a homicidal maniac. Here, the maniac is a local geek with a fatal flaw - he has to take credit for work, and he's let his next target get under his skin.
In Mercedes, the maniac in question might remind you of a mega-disturbed Howard Wolowitz given his techno-savvy and odd relationship with his mother. Retired detective Bill Hodges is one click from ending it all, when "Mr. Mercedes" resurfaces after committing the crime Hodges hadn't solved on the clock. It's a fascinating cat and mouse game that builds toward the promise of a tense climax, but abruptly jerks in another direction. I see why now - King plans two more Hodges books. Whether this story will bleed into Book 2 I don't know, but at least I wasn't put off by this book. So the rest of the trilogy is on my radar, for now.