H. P. Lovecraft's The Dunwich Horror by Joe R. Lansdale

In a lonely New England valley, an isolated farming family steeped in strange rites and old gossip produces a grotesquely precocious son whose rapid growth and secret experiments hint at otherworldly parentage; when the son’s bid to seize a forbidden grimoire from a university library ends in violence, an unseen, gigantic entity born of those dark unions begins wreaking havoc on the countryside, forcing a librarian and a handful of scholars to use ancient lore to expose the creature and drive it back from whence it came.