Ancient Enemy by Tiffany A. O'Brien

A college professor studying a site which functions as a megalithic astronomical calendar must come to terms with his personal demons before he can attempt to help save his student from an evil entity. The story begins at America's Stonehenge, also known as Mystery Hill, in North Salem, NH, where unsuspecting college student Mark Welton encounters the first in a long series of inexplicable phenomena. He thinks he's losing his mind. If only it were that easy...