SOMEWHERE, around the time in 1980 when his novel ''Firestarter'' was published, it was written (I think), or said, that Stephen King was at work on a book that was frightening even to him. This was a fairly intriguing rumor, because since 1974, when his first novel, ''Carrie,'' came out, this prolific genius of horror had produced some fairly scary books, among the best of which were ''The Shining'' (1977) and ''The Stand'' (1978).

But then, after that rumor got around, there came a number of books that were unlikely to have fit its description. It could not have been ''Cujo'' (1981), which was effective enough in its account of a killer dog on the loose, but hardly nightmare material. Nor could it have been ''Different Seasons'' (1982), an uneven collection of novellas, or ''Christine'' (1983), the rather slow-moving story of a homicidal car with a couple of frightening moments here and there. And it certainly wasn't going to be his latest book, ''Pet Sematary,'' with its downright silly title, inspired by a place on the edge of a Maine forest where children had gone over the years to bury their dead dogs and parakeets.

Actually, there are a number of things that at first seem silly and innocuous about Mr. King's latest thriller. Along with the name of the book, there are the titles assigned to its three parts, ''The Pet Sematary,'' ''The Micmac Burying Ground'' and ''Oz the Gweat and Tewwible.''

There are all the portentous epigraphs about famous burials and Jesus's raising of Lazarus. And there's the first visit to the ''sematary,'' shortly after Dr. Louis and Rachel Creed and their two children, Ellie (5 years old) and Gage (a year old), arrive in Ludlow, Me. It is out in the woods in back of the colonial house they have just driven all the way from Chicago to settle into. It has little grave markers with messages on them, such as ''Smucky the Cat. He Was Obediant'' and ''Biffer, Biffer, a helluva sniffer/ until he died he made us richer.'' Who in creation does Mr. King think he's scaring with this claptrap?