The other 3 dollar paperback I took a chance on, Mysterium. An archaeological expedition finds a strange green slab in the Turkish deserts. The U.S. government swoops in and claims it (somehow, you'd think Turkey might at least extort something for it), then sets up a research facility around it near a small Michigan town. The physicist they put in charge of examining it does something, and the entire town ends up in a different, somewhat similar world. Soon enough, the inhabitants of that Earth become aware of this town that appeared from nowhere, and you get an uneasy occupation, as people try to adjust in different ways.
The divergence point has something to do with Christian Gnosticism, and Constantine not converting to Christianity. All the talk flew over my head, though the basic point seems to be someone trying to make or reach a better world (as they see it), and not quite managing it. There's also the added effect that their arrival is going to have a considerable impact on the world, even if it's only temporary. If we're going with a Biblical metaphor, the town of Two Rivers ends up being the serpent in the Garden of Eden. Or it's the apple, and the people of that world are their own serpent. Whatever, the Bible isn't my thing.
There's a few subplots, one about a high school teacher who doesn't care about much of anything, and a scholar from this world. There's the one member of the government project still alive, trying to piece things together. His uncle was the physicist, and he's grappling with trying to understand what happened and what his uncle was trying to do. There's the requisite shy, bookish kid that's investigating on his own because his mother doesn't pay him any mind. One of the blurbs describes Mysterium as Arthur C. Clarke mixed with Stephen King, and I can definitely see that, especially the King influence. Which isn't bad. The book is easily readable, if also fairly predictable. I have no idea if the alternate timeline proposed is at all feasible given the changes (would the Black Death really have struck so many times?), but I was willing to roll with it.