When, earlier this year, Joan Didion posed for an ad for the French fashion house Céline, “the fashion Internet,” The New York Times gushed, “quivered in a way it hasn’t at least since Kim Kardashian stripped nude for Paper magazine.” Some of the quivering, certainly, was due to the fact that Didion, who has long since passed into “icon” status, has served as a role model for young writers for years.