The Santaroga Barrier by Frank Herbert (1982) book

Vintage 1982 mass market paperback of Frank Herbert’s standalone science fiction novel The Santaroga Barrier, set in a small California valley town that mysteriously shuts out outside businesses and influences. Psychologist Gilbert Dasein is sent in to investigate why the community is so different, only to find strange “accidents,” a mind-altering local additive called Jaspers, and a society that might be either utopia or dystopia