THE DEATH OF ELVIS: What Really Happened by Charles C. Thompson II and James P. Cole (Delacorte Press: $19.95; 407 pp.). Those Americans who worship Elvis Presley will have a difficult time with this book. They want to know everything about their fallen idol, perhaps to sustain the prayerful belief that he isn't dead. But can even the most devoted want to take a detailed, illustrated trip through Presley's lower intestines as the doctors slice them open during the autopsy? Do they really want to know that, at his death, the bloated, drugged singer was in the bathroom reading an illustrated book of sex and astrology (when Pisces is in this position, then Capricorn should be in that position, see drawing)? And if they don't, who's going to buy the book? This is the rather belated tale of how-we-got-the-story (that Presley succumbed to drugs), written by two men who got it for ABC-TV 11 years ago. The only "news" is the authors' conclusion that Presley suffered a fatal allergic reaction, having taken codeine instead of his customary Diluadid. This is modern theology-as-pharmacology, middle-American style; not how many angels can dance on the head of a pin, but how many, and which, narcotics a dead god has taken.