Reliable source is here (page 81). Imo worth putting on her Wikiquote page; maybe I’ll do that later. (registration wall, but you’ll just have to trust me.)

Flannery O’Connor once received a letter from an old lady who told her that a book of hers, presumably Wise Blood, had “left a bad taste in my mouth.” Her reply to her was " ‘You weren’t supposed to eat it.’ "