E.L. James, the British author of the hugely successful “Fifty Shades of Grey” erotic novels, is celebrating the fictional birthday of her famous male character on Thursday with the release of her new book, “Grey,” which looks set to be an instant bestseller. Publisher Vintage Books said demand for the novel, told from the viewpoint of business tycoon Christian Grey, has been consistent with the other books in the trilogy, which have sold more than 125 million copies worldwide. She was prompted to write it by fans who had asked her to tell Grey’s side of the story of his erotic relationship with virginal college student Anastasia Steele.