Insomnia is about another interwove part of the dark tower in a way. But the main story features in Maine as King loves to do. It is about an aged man that loses his ability to sleep and awakens earlier and earlier to start seeing strange phenomena in his neighbourhood. That combined with elements a Dark tower fan will understand instantly. Stuff happens on levels of epic that can only be read to understand.
A new way of looking at the world for some as some of the other worlds leak in again.
I mean in the stand, Randall Flag is the main antagonist, But he is the soul antagonist in most of the dark tower follow ons. He's been wandering the hidden highways of the worlds between ;p