Rosemary's Baby occult horror novel, The Mephisto Waltz (1969), from a writer who would go on to write bestselling mainstream historical fiction about the Civil War and whatnot. Great covers all around, from both the US and the UK. All but the original 1970 Signet paperback below highlight the nude female form, titillating potential readers about what might reside within. But I'm not surprised to learn the novel (and its 1971 movie adaptation starring horror-film icon Alan Alda) is fairly lackluster and lame; check out The Midnight Room review, in which it's referred to as "late '60s quasi Satanic hooey." Ha!