A British twin?
I just left the boards for a 1973 movie titled The Wicker Man. Many similarities to this story,I think, although I have yet to see it. An apparently normal village hiding some deep atavistic secret. Shock ending, and all that. I don't have a copy of Tryon's novel anymore to check the copyright date, but it sound like he might have been aware of TWM prior to writing his own. Two books of his were highly creditable horror pieces; check out The Other as another good movie adaptation.
"What we have here...is a failure to communicate."