Mile 81 is not a full length novel. It's only 80 pages in print. Amazon calls it a kindle single. That doesn't mean it's not packed with all the real life grit and horror you've come to expect from Stephen King.
There's an abandoned rest stop at mile 81 on the interstate. The local teens hang out there doing the things teens do. Something else is hanging out there now, and it's something you'd do well to avoid. My advices is to read this one in bright sunshine way before bedtime. Very highly recommended.