The erotic novel “Fifty Shades of Grey’’ - dubbed by some as “mommy porn’’ - started life as an underground phenomenon, but as women worldwide gushed about it on Facebook, in hair salons, and across clinking mahjong tiles, the craze grew. Earlier this month, the author E.L. James, a little-known former TV executive, wife, and mother from Britain, lived out a different kind of fantasy, when Vintage Books part of the Knopf Doubleday Publishing Group, bought the world English-language rights to the book, and Universal Pictures and Focus Features spent a reported $5 million for movie rights to the steamy “Fifty Shades’’ trilogy.