Stephen King’s latest novella, “Riding the Bullet” — available only on the Internet — has proved so popular in two days of literary life that several online booksellers could not meet the astounding demand to download digital copies. The e-book racked up 400,000 orders during its first 24 hours for sale. “I’m stunned,” Jack Romanos, CEO of King’s publisher Simon & Schuster, told The New York Times, which reported that readers overwhelmed the sites offering the story. What does all this mean? That the profile of electronic books has been dramatically raised. The King story sold more cyberspace copies than any of his best-selling novels did on their first day. Romanos compared the breakthrough to the revolution with paperback books more than 30 years ago. Critics point out, however, that because of disappointed readers who couldn’t download the story, no one should use the word “revolution” just yet.