This plays out like a well-crafted episode of Columbo, where the viewer (or reader, in this case) knows who the badguy is from the beginning, and the fun of the story is in getting to see the detective sniff him out.
The head sleuth in this instance is retired Detective Kermit William Hodges (or just Bill), who, when we meet him, is bored, slightly depressed, and spends his days watching crummy TV and contemplating suicide. He is jerked out of the dullness of retirement when a taunting letter from a previous un-captured perpetrator arrives in the mail.
Brady Hartsfield (Mr. Mercedes himself [that's not a spoiler, we know it's him very early on]) is one of the most sinister, WTF-is-wrong-with-this-guy villains King has conjured up yet. Brady is only mildly pitiable, and yet, watching his character unfold is fascinating. Also, his relationship with his mother makes ones' skin crawl. King has taken what might have been an overused horror trope—a creepy, murderous psychopath who is also an ice cream man—and makes it work. Brady is clever, but insane; able to keep a happy, upbeat outer appearance, but underneath is boiling and furious at the world.
This is a satisfyingly complex story, largely because of Bill Hodges' choice to go around the law and operate on his own (illegally, as he is no longer officially able to do police work). There is another element of an aging man trying to figure out a young man's world; Hodges is used to the old way of doing things, and criminals have become more technologically advanced than he can keep up with. For this, he recruits the unofficial help of Jerome, a sharp-witted African-American high-schooler in the neighborhood. Their working partnership is an unique one, frequently humorous, and gels together all the better for its unlikely quality.
Mr. Mercedes has no supernatural elements whatsoever, but it is no less chilling in certain parts—these regarding Brady and his psychotic musings. In recent years, King has somewhat moved away from supernatural or horror elements (as least those as the central focus) and started simply telling stories about people, even if the story does involve some bit or piece of the fantastic. Under the Dome, 11/22/63, and Full Dark, No Stars, for example, were all more-so stories about people and characters than they were about the scary things at the edges. The best part about King's writing (and probably the reason he has such an enormous audience of readers) almost always seems to be the real, relatable characters he creates and their interactions with one another that feel so frequently genuine.
This is a smart, snappy little read, and one that I think might be even more accessible than usual for a first-time King reader. Chances are, most folks who pick this one up have read King before, but if you haven't, it's not a bad place to start. His craft is as sharp as ever.